The use of fertilizers has only a slight effect on ETcrop, unless crop growth was pre- viously adversely affected by low soil nutrition delaying full crop cover. Irrigation imposes a greater demand on fertilizer nutrients; adequately fertilized soils produce much higher yields per unit of irrigation water than do poor soils., provided the fertilizer is at the level in the soil profile where soil water is extracted by the plant. The movement of soluble nutrients and their availability to the crop is highly dependent on method and frequency of irrigation.
Plant Population
The effect of plant population or plant density on ETcrop is similar to that of percentage of ground cover. When top soils are kept relatively dry, evaporation from the soil surface is sharply reduced and ETcrop will be less for low population crops than for high population crops.
Durin.g the early stages of the crop a high population planting would normally require somewhat more water than low density planting d.ue to quicker development of full ground cover. In irrigated agric- ulture plant population has been considered of little importance in terms of total water needs.
Tillage
Tillage produces little if any effect on ETcrop unless a significant quantity of weed is eliminated. Rough tillage will accelerate evaporation from the plough layer; deep tillage may increase water losses when the land is fallow or when the crop cover is sparse. After the surface has dried, evaporation from the dry surface might be less than from an untilled soil. Other factors such as breaking up sealed furrow surfaces and improving infiltration may decide in favour of tillage.
W ith soil ripping between crop rows the crop could be slightly set back due to root pruning.
Mulching
In irrigated agriculture the use of a mulch of crop residues to reduce ETcrop is often considered of little net benefit, except for specific purposes such as reducing erosion, preventing soil sealing and increasing infiltration. Crop residues may even be a disadvantage where soils are intermittently wetted; the water-absorbing organic matter remains wet much longer thus increasing evaporation. As a barrier to evaporation it is rather ineffective. The lower temperature of the covered soil and the higher reflected capacity of the organic matter are easily outweighed by evap- oration of the often rewetted crop residue layer. There may be additional disadvantages such as the
increased danger of pests and diseases, slower crop d.evelopment due to lower soil temperatures, and problematic water distribution from surface irrigation. Polyethylene and perhaps also asphalt mulches are effective in reducing ETcrop, when it covers more than 80 .percent of the soil surface and crop cover is less than 50 percent of the total cultivated area. Weed control adds to the succes- ful use of plastic.
- 66 - Windbreaks
Reduced wind velocities produced by artificial and vegetative windbreaks may reduce ETcrop by about 5 percent under windy, warm, dry conditions at a horizontal distance equarto 25 times the height of the barrier downwind from it, increasing to 10 and sometimes up to 30 percent at a distance of 10 times this height. ETcrop as determined by the overel .:3nditions
using the reduced wind speed data is not altered. In most cases shrubs and trees are used and, due to the transpiration of the vegetative windbreak, overall ET may be more.
Anti -treuispirants
The use of anti-transpirants, natural or artificially induced variations in plant foliage properties and soil conditioners to reduce ETcrop continue to interest many investigators, but is still in the experimental stage.
Part II-
APPLICATION OF CROP WATER REQUIREMENT DATA IN IDENTIFICATION, DESIGN AND OPERATION OF IRRIGATION PROJECTSA number of approaches are available for planning optimum use of water resources in
agricultural production. For irrigation projects they are based on translating production objectives into adequate technical planning criteria. This comprises the collection of needed information on water, soils and crops, the preparation of a tentative plan and the search for the optimal plan by analysing modifications of the tentative plan through a staged and step by step procedure. Several stages of planning can be identified which can be broadly divided into the project identification and preliminary planning stage (II. 1), the project design stage (11.2) and project implementation and operation stage (II.3).
Discussions are centred here around the development of basic data on crop water require- ments and irrigation supply. In terms of irrigation supply, each plarming stage requires a certain type of data; those normally used are given in Table 33.
Table 33 Project Planning Stages and. Irrigation Supply Data
Production objectives
inventory of resources average monthly supply present hydrological budget and monthly peak supply
water resources potential identification of irrigable areas choice of production system
preliminary project location and size irrigation supply requirements method of water delivery
preliminary size and cost of main works engineering alternatives - technical, managerial and financial
project size supply, schedules (size,
layout of distribution system duration and interval of
hydraulic criteria supplies)
cropping pattern supply scheduling
method of water delivery
- irrigation methods and. practices capacity of engineering works phasing of project works optimization of water use
review supply schedules supply schedules on evaluate water use efficiency daily basis, daily field evaluate technical and managerial water budgets
supply control
monitor field water balance
improve and adjust system operation establish data collection routines on water, climate, soil, crop
prepare supply schedules on daily basis
Plarming Stage Data Application Data Required
Project identification
Project design
Project operation
- 68 -
1. PROTECT IDENTIFICATION AND PRELIMINARY PLANNING 1.1 INTRODUCTION
At the project identification and preliminary planning stage a comprehensive inventory of available resources is made. On physical resources, this would include surface and groundwater potential, water quality, existing water uses and certainty of supply. For promising areas, soil surveys (scale 1:10 000) are undertaken to delineate the extent and distribution of soil types, together with their chemical and physical characteristics particularly water-related properties such as soil depth, water holding capacity, infiltration rate, permeability and drainage, erosion and salinity hazards. Evaluation of climate would include temperature, humidity, wind, sunshine
duration or radiation, evaporation, rainfall, occurrence ofnight frost, and others, on which crop selection and crop water needs will be based.. Criteria on production potential under irrigation must justify development not only from an agronomic, technical and economic, but also from a sociological point of view. Knowledge of present farming systems, including among others farm equipment use,
social amenities, credit facilities and farming incentives, will therefore be required in selecting a development plan. Infrastructure and human resources must be evaluated including conununications, markets, population, labour and employment.
Based on the knowledge of available resources, the choice of the production system under irrigation must be made. Important parameters are:
Crop selection: Here, in addition to water available, climate and soils, the preference of the farmer, labour requirements and markets among others must be considered. These are 'often site- specific such as limited water available reAtricting high water-consuming crops, unsuitable soils for some crops, limited labour for highly-intensified production and processing, and areaw-ide
marketing constraints. The cropping pattern may need to be adjusted to the available water supply over time.
Cropping intensity: At the field level, frequently cropping intensity does not correspond to that of the project as a whole. Cropping intensity may also vary with time. Early assumptions must be made since this largely governs the acreage that can be irrigated from the available water and the design and operation of the distribution network. This also greatly affects the level of investment.
Water supply level: An.acceptable level of supply, or irrigation norm, must be selected based on a certain probability that water needs for a selected cropping pattern and cropping intensity will be met for each port-ion of the growing season. For instance, available water supply may be expressed as: (i) seasonal irrigation shortage not to exceed 50 percent of the needed supply in any one year and (ii) sum of irrigation shortages not to exceed 150 percent of the needed supply in a 25-year period. Of particular importance are periods when water shortages have a pronounced effect on yields or germination (Table 32). A detailed evaluation of water supply available and water demands over time is therefore required.
Given a certain supply, in turn cropping patterns may need to be adjusted to avoid peak irrigation requirements at periods of high evaporative demand and peak requirements of various crops occurring simultaneously. This must include consideration of dormancy periods, shifting of sowing dates, transplantation practices, shortening of growing seasons, and others. Knowledge of
the crop response to water during the different growthstages will greatly assist in reducing the risk of possible crop failure or yield depression due to periods of limited water supply.
Method of irrigation: Selection of the method of irrigation needs to be made at an early date by evaluating the required investments, water use efficiency, simplicity of use and adaptability to local conditions, erodability of soils, infiltration rates, water salinity and others. The advantage of one method over another is not so much determined by difference in irrigation supply needed or its efficiency but by the adequetcy with which the crop requirements are met.
Efficiency of the system: The efficien.cy of the system in terms of meeting water demands at field level in quantity and time is determined by both water losses by.canal seepage and the way the system is managed and operated.. Size of the project, method of delivery (either continuous, rotation or demand), the physical control facilities in the system, type of management and communications all become important factors. Additional water losses are incurred during field distribution'a-nd application, and farm layout, land levelling and irrigation practices greatly affect water use efficiency at field. level.
Drainage and leaching; Drainage is essential for successful irrigation; without proper d.rainage rapidly rising groundwater levels and soil salinization can result. To avoid salt accumulation in the root zone and related crop damage, the leaching requirement must be determined. Leaching during off-peak water use periods or non-cropping periods will reduce peak water demand and design
capacity of the distribution system. Timing and depth of leaching will d.epend mainly on type of crop, soil, climate, irrigation practices and irrigation water quality.
In formulating the project, a thorough study of the engineering alternatives is required in order that the most appropriate technical, managerial and economical solution is achieved.
Alternative preliminary layouts of the scheme are generally prepared, including size and shape of commanded areas, water level and flow control, and location and size of required engineering works.
Land ownership, natural boundaries, land slope and land preparation including land levelling must be reviewed in relation to this scheme layout. Feasibility of land consolidation, where needed, should be considered from the legal, technical, economic and particularly sociological point of view.
Accurate evaluation of future project operation and water scheduling cannot be made unless pilot projects are operational at or before the planning stage. No scheme functions perfectly the day it becomes operational. Allowance shoul.d be made in the planning and design to account for changes in cropping pattern and intensity, at the same time avoiding any excesses. Refinements of irrigation sched.uling to match crop irrigation needs should be made after the project has been in operation for some years. The type of data normally used at the project identification and preliminary planning stage is average monthly supply and monthly peak supply.