• Tidak ada hasil yang ditemukan

Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol43.Issue1.Feb2000:

N/A
N/A
Protected

Academic year: 2017

Membagikan "Directory UMM :Data Elmu:jurnal:A:Agricultural Water Management:Vol43.Issue1.Feb2000:"

Copied!
24
0
0

Teks penuh

(1)

Strategies for water management in gravity

sprinkle irrigation systems

A.A. Ramalan

a,*

, R.W. Hill

b

aAgricultural Engineering Department, Ahmadu Bello University, Zaria PMB 1044, Nigeria bBiological and Irrigation Engineering Department, Utah State University, Logan, UT 84321-4105, USA

Accepted 17 March 1999

Abstract

The study examined various management strategies implementable for equitable allocation of water among users in a gravity sprinkle irrigation system (GSIS). A computer model was developed and employed to effect allocations under three scenarios or variations thereof, namely, on-demand, rotation, and continuous supply. Historical streamflow records and climatic data for 3 years, 1979, 1986 and 1983, representing normal, dry and wet years, respectively, were used. Equitable water distribution consideration took into account shares of stock for water held by each user in the irrigation company. Allocations were proportionately determined in relation to the numeric strength of the users' stock when the demand for water was constrained by the available streamflow. The developed management strategies were applied to a real-life irrigation project, the Richmond Irrigation Company at Richmond, UT. Pure on-demand strategy proved to be the most beneficial project-wide, costs of augmentation notwithstanding.# 2000 Elsevier Science B.V. All rights reserved.

Keywords: Gravity-sprinkle; On-demand; On-request; Strategies; Water-rights; Users

1. Introduction

The water source for the gravity sprinkle irrigation system provides water as well as the energy for operating the system. No pumping is required to lift water and pressurize the line as is the case in a conventional sprinkler system. The water source at certain mountain top locations is several hundred meters in elevation above underlying farm

* Corresponding author. Tel.: +234-069-50571; fax: +234-069-50563.

E-mail address:aramalan@abu.edu.ng. (A.A. Ramalan)

0378-3774/00/$ ± see front matter#2000 Elsevier Science B.V. All rights reserved.

(2)

lands. Thus, the elevation difference provides the pressure energy needed to deliver irrigation water at the sprinkler nozzle. The system is fed directly with water from rivers and creeks by the use of simple intake structures in open channels. Typically, these run-of-the-river irrigation systems are characterized by absence of any flood detention or storage structures.

In dry years, as a consequence of low snow pack on the mountain, streamflow in rivers and creeks resulting from snow melt in the spring are deficient. Coupled with the high crop water use in the summer, system performance diminishes. Equitable distribution of water among users constitutes a serious water management problem to irrigation companies1. Studies by Kaewkulaya (1980); Bishop and Long (1983); Khanjani and Bush (1983); (Kim et al., 1983) examined methods for water allocation in open-ditch surface irrigation systems. Information is, however, lacking on techniques for water allocation in a gravity sprinkle irrigation system (GSIS).

A study to examine management strategies to adopt to effect such allocations was undertaken. The nine strategies which evolved integrated with hydrological, soil and climatic factors of the area, crop water use, and the amount of shares held by users in company stock for water. As part of the study, a computer model to effect allocation was developed. The model was tested using data from a real-life irrigation project in northern Utah.

2. Objectives

The general objective of the study was to evolve techniques for improving water management in gravity sprinkle irrigation systems. The specific objectives include identifying and evaluating practical management strategies that could be used by GSIS companies to aid water management. In addition, the study was to compare the attributes of the various strategies and quantify the benefits associated with each.

3. Procedure

The water available to users in an irrigation company is a common commodity by virtue of the legal provisions in water rights. Such waters may be either directly from a diversion of surface source or from groundwater. The challenge for management of mutual irrigation companies is to translate share of stock values into realistic water allocations to users that are fair and equitable. Equity is viewed here in relation to individuals getting their fair share of water. Time is an important element in the allocation process because crop production occurs in sequential phenological phases that respond to environmental changes. Water allocated to users must be in amounts and at times

1Users constitute mutual irrigation companies, granted water-rights through an instrument of State Law to

(3)

reasonable to make the commodity usable and effective. Various management strategies, or operational schemes, have been evolved as means to effect delivery to users. Table 1 gives a listing of the strategies used. A description of each strategy is presented below. Except for the continuous supply strategy, the other strategies under on-demand and rotation schemes are modified versions of the system used in open ditch irrigation systems. Allocations based on selected strategy were implemented using the computer model GSIS, developed as part of the study, which integrated soil, water and environmental factors. The model and the submodels are illustrated in Fig. 1(a±d).

3.1. Descriptions of the management strategies used in the study

3.1.1. Pure on-demand (unlimited access to water by all users)

This is a strategy that seeks to allow all users, access to water at all times. The implementation of this strategy is possible early in the season when an ample supply of water is available and crop water requirements are relatively low. Irrigation durations are similarly expected to be low. No intensive policing of the system is desirable.

The manner with which users may connect for service is expected to follow a random pattern. The main constraint anticipated is with the capacity of the system hydraulics to satisfy demand from users needing water within the same time frame.

3.1.2. Users placed on request

This strategy is a modified demand delivery system. It places some responsibility on the user. It requires that he made known his request for water in advance of time of use to allow a sufficient time lag for scheduling and balancing of deliveries and total system flow rates by the project. Because this strategy allocates water subject to constraint in Table 1

A listing of the strategies used for the water allocation studies in gravity sprinkle irrigation system (GSIS) at Richmond, Utah

Strategy Composition

Pure on-demand

Unlimited access to water by all users fields treated as single entities.

Modified on-demand

Users' supply placed on request fields treated as single entities.

One-time delivery fields treated as single entities.

Rotation

Users grouped in rotation blocks contiguous fields as rotation blocks (three blocks)

Supply based on crop category each crop type forms a block (three blocks)

Min±max Ranking system range in shares of stock, each range constitutes a block (four blocks)

Systems-related strategy hand-move and wheel-move lateral system each constitute a block

(two blocks)

Low labour cost schemes Off-farm and on-farm users constitute a block each (two blocks)

Continuous supply

(4)
(5)

supply, the users' request may or may not be honoured at the requested time. Some waiting may be imminent.

The strategy is particularly expedient during the latter part of the season when water shortages are expected and irrigation amounts and/or frequencies are likely to be high.

(6)

Users' requests are likely to be highest during times of critical crop growth stages. While it may be possible to anticipate incoming requests for water, it may not be possible that all requests made for deliveries at specific times will be met.

(7)

3.1.3. One-time delivery

This strategy considers supplying the user his allocation of water in a one-time delivery. The one-time allocation will be estimated based on a quantity of water that would refill the entire root zone of his crop in a one-time delivery. Users, through mutual agreements, may trade water among themselves; management must be notified of such deals in advance of trades.

3.1.4. Users grouped in rotation blocks

Users' holdings are grouped in rotation blocks. Each block is of a pre-determined size (in hectares). Each user/field in the block is allowed access to a fixed continuous flow

(8)

amount and duration. The frequency is usually fixed, but may also be variable. Individual users within each block irrigating at the same time are grouped as rotation units. Users will have allocations figured out relative to their stock shares. Rotations are expected to be rigid and all were to take their flows only when due. One rotation unit made up of one or more blocks received water at any one time.

3.1.5. Supply users based on crop category

This strategy invokes the use of crop categories in allocation, taking into consideration the economic benefits derivable from the array of crops irrigated. Ranking of users' crops was in effect and allocations were to be along specific crop categories.

First to be allocated water were users who grew crops with the highest economic value, and so on, to users who grow economically least-derivable-benefit crops, such as pasture. The strategy is particularly well adapted to periods of diminishing streamflow.

3.1.6. Min±max ranking system

A ranking system was evolved whereby allocations were begun with users who hold the highest number of shares of stock taking all the flow. This group of users was followed by the next lower level of users and finally by users who held the least stocks. When operating under deficit flow conditions, each user at any of the levels will have water for a time period commensurate with his shares.

3.1.7. Systems-related strategy

This strategy relates to the two major gravity sprinkle irrigation systems, the hand-move and the wheel-hand-move lateral systems. Users were classified into the two groups depending on the type of system each operates.

Allocations using this strategy were along system types, in other words, users operating wheel moves will be expected to be irrigating at about the same time. The exact time individuals within system groups are delivered water depended on the overall system hydraulic analysis.

3.1.8. Low labour cost scheme

This management strategy is invoked whereby certain users are to receive their allocations during the low labour cost hours. The scheme differentiates on-farm from off-farm users. Under certain circumstances and locations, the cost of irrigation labour to the farmer is lower during weekday first-shift hours compared to other shifts. The first shift is between 8 a.m. and 4 p.m. The strategy is to make water available to the farmer during the least labour cost hours.

Urban and non-agricultural irrigation stockholders who use water for lawn sprinkling and backyard gardens will have access to water during the second and third shifts on Saturdays and all three shifts on Sundays.

3.1.9. Continuous supply with monitoring

(9)

4. The computer model

The GSIS model is developed for use in mutual irrigation companies that operate under gravity sprinkle systems. The model is comprised of four submodels, the first being the main programme. The second, third and fourth are designated HYDRA, SWBAL and ATTR submodels, respectively. The main programme serves the primary function of controlling programme flow. It also carries out non-repetitive calculations. The submodel HYDRA provides a work area in which routine calculations of crop water requirements and the need for irrigation are performed. The submodel SWBAL maintains the soil water balance sheet for all fields and the ATTR submodel evaluates the attributes of the various strategies.

The model invokes a strategy to implement allocations, then estimates a field service priority, and determines sprinkler discharges based on operating pressures at delivery points. It also uses stored data on soils, crop and climate to compute crop evapotranspirat-ion at various growth stages for maize (field corn), spring wheat, pasture and alfalfa from:

EtˆKcKsEtr (1)

Where Et is the estimated crop evapotranspiration, Etr is the reference crop

evapotranspiration computed from the Jensen±Haise equation.Kc, the crop coefficient,

is the ratio of the actual crop evapotranspiration to reference crop evapotranspiration. The value varies with crop variety or species, time of planting, and growth stages. It is computed by the use of a phenology clock for each crop for the specific Julian day depending on planting, effective cover and harvest dates. The mean crop coefficients,Kcm

used in the model were adapted from (Allen et al. (1988) based on tabular values presented by Wright (1981, 1982); Doorenbos and Pruitt (1977) rev.). In the equation,Ks

is the soil moisture stress coefficient. For an irrigation system in which maximum Et is allowed,Ksˆ1. The coefficient varies with evaporative demand, soil moisture content,

soil and the crop. This study used the model proposed by (Hill et al., 1984).

The Jensen and Haise (1963) temperature-radiation equation for estimating Etrwas

used to program a Datapod (Omnidata International DP 219) to accumulate the calculated Etr on a daily basis. The data used including air temperature and solar radiation were

sampled at Newton, UT (the nearest weather station to Richmond). Thus,

Etr ˆCtCf…TÿTx†Rs (2)

where Etr is the reference crop evapotranspiration (mm), T is the average daily

temperature (8C),Txis the intercept on the temperature axis,Rsis incident solar radiation

(langleys/day), andCtis a temperature coefficient. Values forCtandTxas 0.025 andÿ3,

respectively, were reported in Hansen et al. (1979). Both factors are site-specific. For this reason, they further gave a relationship for estimatingCtandTxfor other areas taking into

account altitude above mean sea level and saturation vapour pressures of water for the warmest month of the year in the location. Allen et al. (1988) gaveCt(in English units) as

0.009 and 0.011, respectively, for valleys with extensive irrigated or wetland areas, and for surroundings of predominantly arid lands. A fifth power polynomial equation was derived that best fit the Datapod data relating Etrwith day of year. The equation was used

(10)

The hydraulic network submodel, HYDRA, simulates pressures in the distribution main pipes, the laterals and sprinkler nozzles. The relationships given in Keller (1986) were used to estimate average lateral pressures and nozzle discharges using data relating to pipe type and size, friction head loss coefficient, and allowable head loss due to elevation changes. It also provided for corrections for environmental and spray conditions.

The submodel also estimates the allocation time for each user to irrigate with the allotted volume of water,qt. The allotted volume was estimated by

qtˆ2:8a d (3)

whereqis equal to field/user flow capacity (lps);tis equal to time to apply the irrigation (h);a is equal to field area (ha) andd is equal to gross depth of irrigation (mm)

The time of water application,t, was related to infiltration rate of the soil in order to prevent surface runoff and/or ponding. Therefore, the application rate, which is only constrained by the soil infiltration rate, was calculated in the submodel using

iˆ3600qs

SmSl

(4)

whereIis equal to application rate (mm/h);qsis equal to discharge per sprinkler (lps);Sm

equal to lateral spacing along mainline, andSlis equal to sprinkler spacing along laterals

(m).

Once allocations are completed, programme execution is transferred to submodel SWBAL, the soil water balance submodel.

The submodel SWBAL maintains a soil±water budget. This is a daily book-keeping process of where water is in the soil/crop environment. The soil±water budget equation is

SWSnˆSWSnÿ1‡Prn‡IRnÿEtnÿDRn (5)

where SWSnis the soil water content at the end of the present day, SWSnÿ1was the soil

water content the day before, Prn, Irn, Etn, and Drn are, respectively, the precipitation

amount, irrigation depth, actual crop evapotranspiration and drainage loss for the present day. Drainage occurred only if SWSnfor any day exceeded the field capacity storage of

the soil. The present day n subscript refers to a specific day of the year. In the development of the submodel, assumptions were made that the soils within the layers found were homogeneous, infiltration occurred at the same rate in fields, there was no upward movement of water into the root zone, all farmers (users) used the correct nozzle sizes on their sprinklers at all times, and the management allowed depletion was 50% of available water in the root zone (Ramalan, 1988).

As soil water storage capacity is tied to rooting depth, the model uses a root growth function to estimate crop root depth in relation to time. The function assumes that root growth progresses linearly from a minimum root depth at the time of planting (or begin growth in the case of alfalfa) to a maximum root depth value at crop full cover.

(11)

estimated using a phenology clock based on growing degree-days proposed by Gilmore and Rogers (1958).

The submodel ATTR is invoked to evaluate attributes of the various strategies, including costs, returns, and benefits. In addition, waiting time with respect to elapsed time before a user receives service, and equitability indices are estimated in this submodel. The monetary benefit was simply estimated from total returns from sale of produce minus cost of production. Returns were estimated as

ReturnsˆPrice…per unit weight† Yield

ha ‡BYP (6)

where unit weight was based on tonnage or kg. In the case of pasture the unit weight was replaced by animal unit month (AUM), as 1 AUMˆ360 kg DM (Scarnecchia and Kothmann, 1982). Yield was based on production per unit area, tonne/ha. BYP was the revenue from sale of bye-products, such as wheat straw. All the costs toward production were partitioned as yield-dependent, constant or fixed, area-dependent, or capital costs. Thus on-farm total production cost per hectare was estimated as

TCˆCy‡Cc‡Ca‡Ck (7)

where TC is equal to total cost/ha;Cyto yield-dependent costs such as of fertilizers and

fertilizing; swathing and baling for alfalfa; and clipping for pasture. Cc is equal to

constant costs, such as the cost associated with land tax;Cato area-dependent costs, such

as water assessment and irrigating costs. Ck is equal to capital costs, such as costs

associated with machinery and tools. Its value depends on the investment cost of the item, interest rate and pay-back period on loan amount.

The submodel ATTR estimated annual cost of capital items using capital recovery factor CRF, thus,

Annual costˆinvestment costCRF (8)

where

CRFˆ i…1‡i†

n

…1‡i†nÿ1

iis equal to annual interest rate expressed in decimal andnis equal to pay-back period for the loan sum in years.

The input data for calculations in this routine were stored in ECON.DAT for the four crop enterprises in the study.

5. Application of the model

5.1. The site

The Richmond Irrigation Company (RIC) project was the site of the study conducted in the summer of 1986. The project, located in Richmond (418430N, 111

8470W) in north

(12)

formation in the year 1860 by a group of farmers (users). The sole purpose of the company was to effect distribution of water among members for irrigation and municipal uses. The source water is by direct diversion from High Creek, near Richmond. The area of the watershed contributing flow in the creek is 26.1 km2. From historical records (1944±1985), the mean discharge at the main diversion gauging station is about 83 lps. The extreme maximum and extreme minimum discharges ever recorded were 14,000 lps and 70 lps, respectively.

The users who are the stockholders in the company, own and operate farms within the project. The profile of stockholders showed that there were 457 of them in the register who collectively owned 6067 shares. The maximum, average and minimum number of

(13)

shares held by stockholders are 229, 13.28, and 1, respectively, and all were acquired as sprinkle irrigation stock. The farms were flood-irrigated until 1970, when construction of the gravity sprinkle irrigation system was begun (USDA ± Soil Conservation Service, Logan, 1974). The RIC had a total of 10,000 ha for which water-rights were granted by the Utah Division of Water Resources through the Kimball Decree Awards #38. The priority date for the first right to the company was issued on 1 May 1860. Three other irrigation companies in the vicinity which also have water-rights on the High Creek are Coveville, Webster, and Mt. Home.

The interpretation of these awards (Ramalan, 1988) which are in three parts gives RIC the right to divert 78.4% of the flow in High Creek for all flows between 141.2 lps (5 cfs) and 1412 lps (50 cfs). If the flow in the creek is greater than 1412 lps (50 cfs) and up to 2613 lps (92.2 cfs), RIC has an additional award to divert 20.2% of flow that is in excess of 1412 lps (50 cfs). At this 2613 lps (92.2 cfs) flow, which is regarded as maximum ditch flow, the company by virtue of the two awards has rights to divert a total of 1338 lps (47.2 cfs). The third award comes into effect if the flow in the creek exceeds 2613 lps (92.2 cfs). The third award allows the company an additional 20.2% subject to an upper ceiling of 1984 lps (70 cfs). At the time of study the RIC had not operated any ground water rights.

5.2. Pumping

Four electrical deep well turbine pumps exist within the project. The pumps with rated discharges of 60 lps (900 gpm) at heads 80 m (260 ft) were to pump from 30 cm diameter, 98.8 m deep wells. The pumps were located in a fenced pump house in sector four close to the Logan±Richmond highway. The location was a few hundred meters from the pipe intake structure across the road. The pumps feed the project mainline which in turn feed distribution lines servicing different farmers' fields. The pumps were not designed to serve individual fields rather they feed into the common pipelines arranged in branching hydraulic network. The study took cognisance of these facilities to include the possibility of need for flow augmentation by use of ground water.

5.3. Procedure for estimating augmentation and costs

The general procedure used to estimate augmentation requirement and its costs, especially in relation to strategies 1 and 8 is summarized below. The energy type was electricity and a Utah Power and Light Company (UPLC), schedule of charges used. Augmentation requirement was estimated

Qaug ˆDQJdÿHQJd …if Qaug>0† (9)

and

Vaugˆ8:6410ÿ3Qaug (10)

whereQaugis equal to the required flow augmentation in lps; DQJdis equal to the demand

flow on a particular Julian date in lps; HQJd equal to historical streamflow for the

(14)

It was anticipated by the project designers that augmentation, if needed, would not exceed 226.7 lps (8 cfs) even in dry years. Such need is to be met by the output from the four pumps installed. Each pump was rated at 56.67 lps (2 cfs) and powered by 75 hp motors connected to three-phase, 415 V power source. The number of pumps needed to supply augmentation on any day was estimated by (rounding up to next integer)

PUMPNˆ Qaug

56:67 (11)

i.e., one pump if 0<Qaug56:67 lps; two pumps if 56:67 lps<Qaug113:33 lps; three pumps if 113:34 lps<Qaug170:00 lPs; and four pumps if 170:00 lps<Qaug

226:66 lps; where PUMPN is equal to number of pumps operating.

The electric power requirement for the pumps to supply the augmentation flow was estimated

Powerˆ Qaugh

56:6Ep

(12)

where Power is equal to required power of electric motor in kW;h to pumping depth in m;Epis equal to pump efficiency (expressed in decimal) andQaugas previously defined.

The energy requirement was calculated as

EnergyˆPowerTime (13)

where Energy and Time were measured in kW h and h, respectively. Both power and energy were tallied to obtain monthly and seasonal totals. Monthly power and energy costs were estimated using option A of the UPLC schedule. Monthly bills for augmentation were estimated in the model by

MNBILLˆFXCOST‡SERVCHG‡ …MISCOST‡DEMCOST‡ENECOST†mn

(14)

where MNBILL is equal to monthly bill in US$; FXCOST is equal to fixed cost representing amortized costs for wells, motors and pumps, and shelter based 30, 20, and 10 year lives, respectively, at 12% interest rate. All cost details were obtained from the local equipment dealer. SERVCHG is equal to service charge per connection by the Utility company. MISCOST is equal to miscellaneous costs, which are associated with attendance, lubrication and repairs on per 100 h basis for the month. DEMCOST is equal to UPLC monthly power demand cost based on $8.04/kW, first 100 kW; and $5.33/kW all additional kW; ENECOST is equal to UPLC monthly energy cost based on 10.0345 cents/kW h first 100 kW h per kW; 5.3803 cents/kW h next 25,000 kW h; and 3.5702 cents/kW h all additional kW h, and mn is subscript referring to the specific month.

(15)

Seasonal bill is a summation of monthly net bill minus off-season costs. The off-season costs are equipment costs for which payment must be made during and off-irrigation season. These data were entered in a data file, AUGMENT.DAT for use in the model.

5.4. Farm and field characteristics

The RIC project is in four sectors. Each sector is served by an intake structure that feeds water into the mainline for the fields in the sector. The data collected for this study were gathered in the fourth sector which has an area of 251 ha.

For at least the purpose of this study, a field was considered a piece of land with unique crop, soil, and physical characteristics. The field was, therefore, expected to consist of one crop on essentially one soil type. The field had physical boundaries based on either soil or crop differentiation. One or more fields constituted a farm. Records of field sizes obtained through direct interview with the farmers varied from 2 to 32 ha with a mean of 11 ha.

Soil types defined by textural classification given in the soil survey data of the area showed that Trenton silty clay loam was the most common. About 70% of the area under this soil type is cropped with irrigated crops such as alfalfa, small grains, sugar beet, and pasture. This soil and others elsewhere on the site are moderately well drained though slowly permeable. Runoff is low and hazards to erosion slight. Water-table depth is commonly more than 100 cm.

Crops in the project fall within the categories of irrigated or dryland commercial crops. Their distribution among fields, however, varied with climatic and soil conditions, economics and farmers preferences. For the 1986 crop year, aerial distribution of crops among fields in the study area was as follows: alfalfa, 65%; small grain, 13%; silage corn, 9%; and pasture, 13% on land areas of 181, 14, 24, and 32 ha, respectively. About 177 ha, out of the 251 ha, were on Trenton silty clay loam and Nibley silty clay loam soils.

Water for irrigation was distributed through a branching pipe network. The irrigation systems in the farms are sprinklers of either hand-move or the periodic wheel-move systems. Water channelled from the creek in open ditches was fed to the pipe inlet at canal turnout about 60 m in elevation above the lowest farm delivery points. Most delivery points were located about 52 m below the elevation of canal turnout. Excess pressures not used in the pipeline were dissipated at pressure relief valves strategically located within the network. Pipe sizes of standard lengths of 6 m made from welded steel or asbestos cement and fittings were joined and buried to constitute the network.

At delivery points, the network terminated to 100 mm stand valves equidistantly spaced on laterals along fence lines of individuals' properties. Valve openers facilitated release of water into individual sprinkler systems.

5.5. The model implementation

(16)

the study area, as allowed for by the water rights, varied from 14 lps to a maximum of 178 lps. Daily streamflow data adjusted with water-right awards factor (as a percentage) represents the allowable diversion due to RIC. These daily estimates were stored in an array variable HQ in data files CLIM.DRY, CLIM.NOM and CLIM.WET. They were read for each computer run. Recorded precipitation are also included on a daily basis as part of the input files. Each computer run called the relevant subroutine after having previously opened and read all input data files. All the calculations carried out in the sub-routines are written in output data files before exiting to the main programme.

6. Results and discussions

For the test years, with allocation on demand, the study showed that it was infeasible to operate the strategy for allocation without augmentation throughout the growing season. This is evidently so because demand flow rates on the system outstripped supply in some instances, as depicted by the hydrographs in Fig. 3 particularly in the flow receding region where demand-to-supply ratio exceeded 8 : 1. In the normal and wet years, it was possible to operate on-demand from day of year 101 through Day 135 and Day 175, respectively. Within these periods demand flows could easily be satisfied by supply. But the supply hydrograph for the dry year showed significant deficit as to make allocations on-demand inoperative. Scanty precipitation coupled with low stream flows preclude operating on-demand except intermittently for some 10 days throughout the growing season. The model showed that in addition at least one pump needed to be operated continuously.

Beyond those periods when on-demand allocations were feasible in the normal and wet year, constraints in delivery were inevitable. From these days onward, on-demand was viable only to the extent that water supply was augmented through conjunctive water use by tapping groundwater sources. The total costs for augmentation in order to continue to operate on-demand were $44,679, $96,257, and $144,735 in wet, normal and dry years respectively. A substantial proportion of this cost was for energy for pumping. In the only alternative, as would be the case for most run-of-the-river GSIS with no groundwater rights, the schedule of allocations has to change to a less flexible system such as rotation or continuous supply.

The data on seasonal crop water use for the test years for the three strategies are given in Table 2. The average seasonal actual crop Et's were 760, 510, 580 and 710 mm for alfalfa, corn, small grain, and pasture, respectively. The number and net depths of irrigation were highest under the continuous supply strategy. Drainage, estimated by the model as the component of applied water or rain that had percolated below root zones, was least with the on-demand mode, where values exceeded 25 mm per season. These drainage amounts were mainly in corn and grain fields. The seasonal rainfall received in alfalfa and pasture fields was about 280 mm with about 178 mm on the corn and small grain fields.

(17)

Fig. 3. Available and demand flow hydrographs for irrigation on-demand, precipitation, number of pumps for supply augmentation, and number of users irrigating based on Dry Year (1979) data, RIC.

Ramalan,

R.W

.

Hill

/

Agricultur

al

W

ater

Manageme

nt

43

(2000)

51±74

(18)

Table 2

Summary of water use (seasonal totals) by field/user during a normal year with the on-demand, rotation, and continuous supply strategies invoked Field/

CARI430 ALFALFA 16 281.9 3 408.9 0.0 61.0 754.4 10 449.6 147.3 170.2 754.4 16 457.2 137.2 152.4 754.4

CAR1593 ALFALFA 16 281.9 3 414.0 5.1 61.0 751.8 10 574.0 236.2 132.1 751.8 16 586.7 223.5 106.7 751.8

CARI688 S/GRAIN 16 188.0 6 464.8 83.8 15.2 584.2 10 398.8 243.8 157.5 500.4 13 419.1 248.9 160.0 518.2

CARI836 PASTURE 20 289.6 7 495.3 55.9 2.5 731.5 10 355.6 162.6 127.0 609.6 16 363.2 152.4 127.0 627.4

CARI525 PASTURE 4 281.9 6 416.6 45.7 66.0 718.8 10 566.4 279.4 132.1 701.0 16 574.0 266.7 129.5 718.8

CARI750 S/GRAIN 8 200.7 6 472.4 91.4 2.5 584.2 10 317.5 208.3 154.9 464.8 13 325.1 203.2 157.5 482.6

CARI142 PASTURE 8 289.6 7 487.7 58.4 15.2 734.1 10 599.4 297.2 119.4 708.7 16 599.4 287.0 124.5 729.0

CARI950 ALFALFA 32 281.9 2 436.9 0.0 ÿ27.9 690.9 10 66.0 0.0 208.3 558.8 16 68.6 0.0 208.3 558.8

CARI011 ALFALFA 6 281.9 3 408.9 2.5 63.5 751.8 10 571.5 221.0 119.4 751.81 16 581.7 218.4 106.7 751.8

CARI066 ALFALFA 12 281.9 3 403.9 10.2 76.2 751.8 10 569.0 226.1 129.5 751.8 16 584.2 228.6 114.3 751.8

CARI122 ALFALFA 6 289.6 3 406.4 15.2 68.6 751.8 11 594.4 236.2 101.6 751.8 16 599.4 233.7 91.4 751.8

CARI474 ALFALFA 12 289.6 3 424.2 17.8 53.3 751.8 11 594.4 238.8 104.1 751.8 16 609.6 238.8 88.9 751.8

CARI299 ALFALFA 12 289.6 3 408.9 7.6 61.0 751.8 10 576.6 228.6 114.3 751.8 16 581.7 221.0 101.6 751.8

CARI869 ALFALFA 14 281.9 3 408.9 2.5 63.5 751.8 10 294.6 78.7 208.3 706.1 16 302.3 76.2 208.3 716.3

CARI958 CORN/SL 10 172.7 3 317.5 5.1 33.0 518.2 10 452.1 137.2 12.7 502.9 14 533.4 182.9 7.6 528.3

CARI481 ALFALFA 12 281.9 4 510.5 17.8 ÿ22.9 751.8 10 444.5 149.9 177.8 751.8 16 411.5 129.5 188.0 751.8

CARI986 S/GRAIN 4 175.3 6 467.4 76.2 15.2 581.7 10 825.5 538.5 119.4 581.7 13 777.2 487.7 119.4 581.7

CARI399 CORN/SL 4 172.7 4 396.2 50.8 5.1 525.8 9 538.5 200.7 5.1 515.6 14 510.5 175.3 5.1 513.1

CARI640 ALFALFA 4 281.9 3 406.9 0.0 68.6 756.9 9 388.6 94.0 177.8 756.9 16 363.2 81.3 193.0 756.9

CARI059 ALFALFA 10 292.1 3 411.5 22.9 71.1 749.3 10 289.6 88.9 193.0 685.8 16 279.4 88.9 193.0 675.6

CARI618 ALFALFA 20 297.2 2 340.4 0.0 119.4 756.9 9 396.2 106.7 170.2 756.9 16 370.8 91.4 180.3 756.9

CARI181 ALFALFA 2.8 281.9 3 403.9 2.5 68.6 754.4 9 490.2 165.1 144.8 754.4 16 457.2 139.7 152.4 754.4

CARI714 ALFALFA 2 289.6 3 401.3 12.7 71.1 751.8 10 619.8 261.6 101.6 751.8 16 581.7 223.5 101.6 751.8

Note: NET IRRG: applied water that infiltrated the soil surface; RT-Z DEPL: root zone depletion.

(19)

crops were not water-stressed, average benefit to users who engaged in the following crop enterprises were as follows: alfalfa, $746/ha ($44/tonne); silage corn, $832/ha ($15/tonne); small grains, $281/ha ($474/tonne); and pasture, ÿ$84/ha (ÿ$14/tonne). With constrained allocations, results showed that benefits accruing to users are lower and indeed some alfalfa fields returned negative benefits in the dry year. The aggregated benefits resulting from the use of each strategy are included in Table 3 for the test years.

The relative ratios of user allocations and stock for water held in the company were compared to draw inferences regarding equitability of allocations among users. The results given in Table 2 for the normal year using the strategies showed that some users received more or less water than their shares of stock would otherwise entitle them to. In the on-demand mode no restrictions were imposed, each field received as much water as it needed. Those fields/users that got far less relative to their shares had far more shares than they needed. While no absolute agreement between the ratios was expected and obtained with on-demand, the non-flexible strategies attempted to ration water based on shares of stock.

Waiting time or stress-days for this study was calculated based on number of days since 50% available soil moisture storage was depleted. It varied with strategy used and the test year. For the on-demand strategy waiting time in the dry, normal and wet years were 11, 7, and 5 days, respectively. Waiting time for other strategies were also estimated. The durations varied between 28 and 101 days. Waiting time with continuous supply was shortest, because users received water on a daily basis. Waiting time for all strategies were longest during the dry year and shortest in the wet year. There was evidently a relationship between waiting time and crop yield achieved. It appeared that the longer the waiting time with a given strategy, the more the individual fields are stressed and the less the crop yields and potential benefits to users.

The resource utilization ratio (RUR), provided a measure of how closely total allocations to users related to available water resources at the disposal of the company. In the dry season, RUR amounted to 1.84, thus users required 84% more water in volumetric terms on a seasonal basis than they had water rights for in order to operate on-demand. The specific times to inject additional water can be seen on observation of demand and supply hydrographs, given as Fig. 3. In normal and wet years, the ratios potrayed a standpoint of sufficiency in meeting seasonal demands; this is to be realized only if assumption of trading surplus water at the beginning of the season holds. If not, augmentation from groundwater had to be in effect. To match allocations closely with what was available, the ratios had to be less than 1.0. The data on Table 3 showed that RUR ranged 0.32 and 0.94 for the other non-flexible strategies. With one-time delivery strategy, where only one heavy irrigation was given to refill the root zones, in the normal year just about 35% of the streamflow was utilized. Augmentation was needed only if streamflow was low to satisfy the commitment for the one-time delivery.

(20)

Table 3

A summary of attributes for management strategies implemented in the dry, normal, and wet years (1979, 1986 and 1983), respectively

Unlimited access to waterc dry 143464 11 80.9 103.0 1.84

norm. 153961 7 72.0 67.2 1.00

wet 154331 5 87.2 28.9 0.79

Unlimited access to waterd dry ÿ5214 11 80.7 103.0 1.84

norm. 51111 7 74.3 67.2 1.02

wet 105509 5 89.0 28.9 0.80

Modified demand

User on request dry 64268 74 90.7 0 0.91

norm. 89183 58 72.0 0 0.52

wet 136231 34 92.6 0 0.63

One time deliverye dry 5294 101 46.6 24.3 0.71

norm. 84175 67 47.8 0.85 0.35

wet 117441 65 47.4 0.13 0.63

Rotation

Users in rotation blocks dry 34203 79 84.0 0 0.84

norm. 110330 38 126.5 0 0.91

wet 127400 29 130.4 0 0.89

Among crop categories dry 19651 91 83.5 0 0.83

norm. 105760 43 123.8 0 0.89

wet 125414 37 127.4 0 0.87

Stock ranks dry 28757 81 86.0 0 0.86

norm. 109227 40 127.6 0 0.92

wet 128314 35 132.1 0 0.90

System types dry 28280 83 82.4 0 0.82

norm. 106015 42 126.5 0 0.91

wet 125889 38 131.0 0 0.89

Labor cost scheme dry 21426 90 84.6 0 0.85

norm. 103073 43 125.7 0 0.90

wet 123639 38 131.3 0 0.90

Continuous supply

Continuous supply dry 23841 80 89.6 0 0.89

norm. 102651 35 130.1 0 0.94

wet 117763 28 135.6 0 0.93

aWater supplied, measured in ha m.

bResource utilization ratio (RUR), defined as the ratio of water supplied (stream plus augmentation) and

allocated to users to streamflow resource available to project for which water-right is granted.

cAssumes unlimited access to water, with no cost supply augmentation during times when flow demand

exceeds available supply hydrographs.

dAugmentation costs included were $144,735, $96,257, and $44,679 for dry, normal, and wet years,

respectively.

eAugmentation costs included were $38,405, $5,665, and $4,479 for dry, normal, and wet years,

(21)
(22)

Table 4 (Continued)

June 9.46 432 19336 14502.15 554.52 13947.64

July 21.85 744 44663 31799.81 1208.81 30591.01

August 32.83 744 67117 46976.20 1788.87 45187.34

September 3.02 216 6177 5317.71 214.56 5103.15

Total vol. of augmentation (ha m)

67.2 0 0 0

Total aug. cost $96,257.00 0 0

(23)

to the project. On-request, because of its bias toward the more profitable crops and enhanced productivity, returned higher benefits to the project relative to the amount of water supplied. The higher productivity was, however, at the expense of equity. One-time delivery performed almost as well as on-request in the normal year. The one heavy dosage of irrigation given to refill the root zone was not beneficial to grain crops in particular as some fields failed, more so in the dry year. Continuous supply performed no better than any of the modified rotation strategies; it had slightly lower average waiting time in the dry year.

7. Conclusions

The study addressed a felt need in run-of-river gravity sprinkle irrigation system. The strategies developed for managing water allocations using the GSIS computer model provides a valuable management-decision tool for simulation as well as operation.

In implementing allocations in normal and wet years, on-demand strategy is an obvious choice since emphasis are on higher monetary benefit to the project and less waiting time. No attempt should be made to operate on-demand in the dry years. At these times, it is incumbent on the project to deliver water to all users to satisfy the legal provision of the water-rights. To such end, continuous supply or rotation strategies facilitate allocations more equitably, though at the expense of achieving high productivity. Augmentation in a dry year is not advisable because it results in financial loss unless cheaper energy for pumping is available.

Acknowledgements

The work reported here enjoyed partial support from the Utah Division of Water Resources to which we are gratefully appreciative. Gratitude is also expressed to the President and several members of the Richmond Irrigation Company for granting unlimited access to their lands for data collection and to the staff of USDA ± NRCS (formerly SCS) Cache Valley office in Logan for assisting us with construction drawings of the pipe network. Finally, the skill and patience of Becky Andrus for finalizing the figures and equations and Peggy Shumway for retyping the revised manuscript is greatly appreciated.

References

Allen, L. Neil., Hill, Robert W., 1988. Crop irrigation requirement model for Utah. Proceedings: Planning Now for Irrigation and Drainage in the 21st Century. Specialty Conference of the Irrigation and Drainage Division. ASCE, Lincoln, NE, pp. 724±731.

Bishop, A.A., Long, A.K., 1983. Irrigation water delivery for equity between user. J. Irrig. Drainage Eng. ASCE, vol. 109(4). Proc. Paper 18454, December, pp. 349±356.

(24)

Gilmore Jr., E.C., Rogers, J.S., 1958. Heat units as a method of measuring maturity in corn. Agron. J. 50, 611± 615.

Hansen, Vaughn E., Israelsen, Orson W., Stringham, Glen, E., 1979. Irrigation Principles and Practices, fourth edn. Wiley, New York, 417p.

Hill, R.W., Hanks, R. John., Wright, James, L., 1984. Crop yield models adapted to irrigation scheduling programs. Research Report 99, Utah Agricultural Experiment Station. Utah State University, Logan, Utah. Jensen, M.E., Haise, H.R., 1963. Estimating evapotranspiration from solar radiation. Am. Soc. Civil Eng. Proc.

89 IRA, pp. 15±41.

Kaewkulaya, J., 1980. Scheduling rotation irrigation for multiple crops in a large scale project. Ph.D. Dissertation. Utah State University, Logan, UT (unpublished).

Keller, Jack, 1986. Sprinkle and Trickle Irrigation Lecture Material. Agricultural and Irrigation Engineering Department, Utah State University, Logan, UT, Winter Quarter, 621p.

Khanjani, M.J., Bush, J.R., 1983. Optimal irrigation distribution systems with internal storage, transaction. Am. Soc. Agric. Eng. 26(3), 743±747.

Kim, S., Busch, J.R., Yoo, K.H., 1983. Predicting daily irrigation project diversions. Research Technical Completion Report, Project A-074-IDA. Idaho Water and Energy Resources Research Institute, University of Idaho, Moscow, Idaho, October, 74p.

Ramalan, A.A., 1988, Management Strategies for Gravity Sprinkle irrigation Systems, Ph.D. Dissertation. Utah State University, Logan, Utah, 340p (unpublished).

Scarnecchia, David L., Kothmann, M.M., 1982. A dynamic approach to grazing management terminology. J. Range Manage. 35(2), 262±264.

Wright, J.L., 1981. Crop coefficients for estimates of daily crop evapotranspiration. Irrigation Scheduling for Water and Energy Conservation in the 80s, ASAE. Proceedings of 1981 Irrigation Scheduling Conference. Chicago, IL, pp. 23±81.

Referensi

Dokumen terkait

Oleh karena itu dalam kaitannya dengan topik di atas diajukan pertanyaan: apakah kerja, dalam hal ini wirausaha, dipandang sebagai suatu keharusan hidup, atau sesuatu

Kebijakan Keterbukaan Informasi Publik belum sesuai dengan pelaksanaan pelayanan informasi publik di Kementerian Pertanian, hal ini dapat terlihat pada level operasional

Faklor pertama adalah rumpur pakan yang terdiri dari Brachiaria deutmhens (Rl), Eleusine indica (R2), Panicun ma-rimunt (R3), Pennisetum pulpureum (Pt4), Setaria sphacelara

11Skala Penafsiran Skor Rata-Rata Variabel Y (Loyalitas Kerja) Error.. Bookmark

Sedangkan di wilayah regional Indonesia Timur seperti Sulawesi dan Papua relatif lebih kecil, yaitu 51% (Wendyartaka, 2016). Terkait penentuan status air sungai tercemar

Dengan demikian Ilmu dan teknologi Kimia, melalui pendekatan kimia hijau dapat membuat aspek-aspek ini dikembangkan dan dikelola dengan lebih berkelanjutan, yaitu dengan

dan pengesahan ormas yang didirikan oleh warga negara asing sebagaimana dimaksud dalam Pasal 34 sampai dengan Pasal 36 diatur dengan Peraturan Pemerintah

Meski angka kejadian komplikasi terus membaik, pendarahan masih merupakan komplikasi paling sering yang terjadi selama prosedur TURP, terutama pada prostat dengan