Extraterrestrial radiation (Ra) expressed in equivalent evaporation in mm/day 25 Average daily duration of maximum possible sunshine hours (N) for various. Prediction of ETo based on the Blaney-Criddle f-factor for various conditions of minimum relative humidity, sunshine duration and daytime wind.
To calculate the solar irradiance (Rs) from sunshine duration or cloud cover data, to determine the weighting factor (W) from temperature and altitude data and to select the appropriate adjustment as given by the relationship between W.Rs and ETo in Figure 2 for different average humidity and wind conditions during the day, the following procedure is suggested. See for example: Data on solar radiation and radiation balance; Routine observations for the entire world.
JF MAMJ JASON
Rn = net radiation in equivalent evaporation in mm/day f(u) = function related to wind. ea-ed) the difference between the saturated vapor pressure at the mean air temperature and the mean actual vapor pressure of the air, both in mbar. Values (1-W) 1.1 W =6 /(A ), where A is the rate of change of saturated vapor pressure with temperature.
J J A SON D
Also the color of the pan and the use of screens will affect water losses. Turbidity of the water in the pan does not affect Epan data by more than 5 percent.
SELECTION OF CROP COEFFICIENT
A large range of kc values in the initial growth stage after sowing is shown in Figure 5. Below are the steps required to obtain kc values for the various stages. Assume a straight line between the kc values at the end of the mid-season and at the end of the growing season;
The kc(low) values are 10 to 20 percent higher than the kc(low) values shown for alfalfa, as they are substantial. Pasture (grass, stubble and alfalfa): kc values will vary widely depending on grazing practices. In the last two cases, the kc values should be reduced when the soil coverage is less than 35 percent.
For paddy rice, kc values are given in Table 28 for different geographical locations and seasons. In Table 30, the kc values for different aquatic weeds are given for various climatic conditions.
CULTURAL PRACTICES Fertilizers
In the early stages of the crop, a high population planting would normally require slightly more water than a low density planting due to the faster development of full ground cover. In most cases, shrubs and trees are used and the total ET may be higher due to the transpiration of the vegetative windbreak. A production irrigation system should be selected based on the knowledge of the available resources.
Crop Selection: Here, apart from available water, climate and soil, farmer preferences, labor requirements and markets among others must be considered. Cutting intensity: At the field level, cutting intensity often does not correspond to that of the project as a whole. Early assumptions must be made as this largely governs the area that can be irrigated by available water and the design and operation of the distribution network.
For example, the available water supply can be expressed as: (i) the seasonal irrigation deficit should not exceed 50 percent of the required supply in a given year and (ii) the sum of the irrigation deficits should not exceed 150 percent of the required supply in a year. year period. The size of the project, the mode of delivery (continuous, rotational or demand), the physical control facilities in the system, the type of management and communication all become important factors.
SEASONAL AND PEAK PROJECT SUPPLY REQUIREMENTS
Irrigation method: The choice of irrigation method should be made at an early date by evaluating the necessary investments, water use efficiency, ease of use and adaptability to local conditions, soil erodibility, infiltration rate, water salinity, etc. . System efficiency: The efficiency of the system in terms of meeting water demands at the field level in quantity and time is determined both by water losses from canal drainage and by the way the system is managed and operated. Flushing during off-peak water use periods or non-planting periods will reduce peak water demand and design.
In project formulation, a thorough study of engineering alternatives is required in order to achieve the most appropriate technical, managerial and economic solution. Preliminary alternative layouts of the scheme are generally prepared, including the size and shape of the command areas, water level and flow control, and the location and size of engineering works required. Net crop irrigation requirements (In) are calculated using the field water balance.
The leaching requirement (LR) is the portion of the applied irrigation water that must drain through the active root zone to remove accumulated salts. For preliminary planning, the capacity of the engineering works can be obtained from the supply required during the month of peak water use (Vmax).
FMAMI T A SOND
FIELD AND PROJECT SUPPLY SCHEDULES
For the planning and operation of the water distribution system, the requirements for the supply of individual fields will have to be expressed in flows or the size of the flow (q in m3/s) and the duration of the supply (t in seconds, hours or days). The capacity and operation of the distribution system is based on the supply needs during the month of peak water consumption. The depth of water immediately available to the crop is defined as p.Sa, where Sa is the total available soil water (Sfc - Sw) and p is the fraction of the total available soil water that can be used by the crop without this affected his ev. -apotranspiration and/or growth.
With rotary supply, the capacity and operation of the distribution system is based on a constant or fixed supply to each field or farm (q) while the supply duration (T) and supply interval (1) vary according to the changing irrigation requirements of the field . the growing season. Evaluation of system design and operation criteria should begin with an analysis of field irrigation requirements. The performance of the distribution channel supplying the individual field or farm is expressed in the supply duration (T) and supply interval (I), which in turn are based on the supply required at the field level (q.t) and the field application interval . (i) (2.2.2).
At the design stage, a number of assumptions must be made; several alternatives in the operation of the supply system and field irrigation plans should be considered. Determine peak daily supply requirements (Vmax) and average daily supply requirements (Vi) for each part of the crop.
REFINEMENT OF FIELD SUPPLY SCHEDULES
The services of an agro-meteorologist will be required to select the equipment and sites, to train personnel, to advise on observation programs and to analyze the data obtained. A detailed land survey (scale 1:5 000 to 10 000) should have been completed prior to the design of the project. Additional investigations will be required, especially on physical and chemical properties of the soil and their changes under long-term irrigation. 2/ Soil salinity and groundwater table observations must be made at regular intervals.
Monitoring of the project must be carried out continuously and include evaluation of the method of supply and planning of water for irrigation, as well as studies of the efficiency of water use with direct measurements of individual components. Modification and rehabilitation of systems should be considered as an integral part of long-term irrigation development planning. Soil data on water retention characteristics; crop rooting depth and the rate of maximum water depletion in the soil.;.
Additional savings can be made by allowing the soil to dry to the maximum allowable rate at the end of the flowering season, rather than leaving a high level of available soil water at harvest time; perhaps an irrigation or two can be saved by this practice. Also, K values may be similar for similar climates, but the effect of different growing season lengths and the nature of the ETo/f relationship will present interpolation problems.1/ The latter is shown in Figures 2a and 2b .
MJJ ASO N D
Ratios for ETgrass and W.Rs for ten sites Values of 'b' for three of the 16 ratios for RHmean and Uday ranges. The evidence of many published relationships shows that it is necessary to take into account in particular the effect of wind for different climates (Rijtema, 1965; Aboukhaled et. a single function that is useful in different climatic conditions and easy to use, f(u) was calculated for data sets that were sorted by mean wind speed (u) and f(u) To - W. Rn- T÷E1 - WXea - ed..1/ The results for three locations are given in Figure 5 and for 9 locations in Figure 6.
The effect of the way (ea - ed) in mbar is calculated is shown by an example of 20 days at Davis (Figure 7). The spread of f(u) relations indicates a serious limitation of the application of the Penman method to a wide range of conditions, although some of the spread may be due to experimental error, techniques used, and advection. To avoid local calibration and simplify the use of the Penman method, a single linear relationship was chosen, i.e.
Only general indications of the accuracy of methods to predict ETo can be given for different climates as no baseline climate type exists. In the case of the 6.1 meter diameter weighing lysimeter at Davis, during the years 1959-63. available soil water was.
COMPUTER PROGRAMME F612. ESTIMATION OF REFERENCE CROP EVAPOTS.ANSPIRATIÒN
AVAILABLE SOIL WATER, Sa: the depth of water stored in the root zone between the field capacity (Sfc) and the wilting point (Sw); nun/m soil depth. EVAPOTRANSPIRATION: the rate of water loss through transpiration from vegetation plus evaporation from the soil; mm/day. FIELD APPLICATION EFFICIENCY, Ea: the ratio of water directly available to the crop to that received at the entrance of the field.
NET IRRIGATION DEMAND, In: the depth of water required to supplement evaporation minus the contribution from precipitation, groundwater, stored soil water; does not include operating losses and flushing requirements; mm/period. EQUIPMENT EVAPORATION Epan: rate of water loss by evaporation from the entire open water surface of a pan;. SC:-.1L 'HYDRAULIC CONDUCTIVITY, k: rate of flow of water through a unit cross section of soil under a unit hydraulic slope; also called permeability or transmission; nun/day.
STORED SOIL WATER, Wb: the depth of water stored in the root zone as a result of previous rainfall, snow or irrigation applications and which fully or partially meets the crop's water needs in subsequent periods; TRANSPIRATION: rate of water loss by the plant, regulated by physical and physiological processes; rnm/day.
