Depending on the production function, irrigation water cost function and the sale price of yield, previously, lengthy mathematical expressions have been developed to estimate the optimal levels of water used in deficit irrigation, that would maximize yield (W m), under water limiting (W w), and under land limiting (W l) and the amount of water that leads to income equal to income of W m when land is limited (W el), and that leads to farm income equal to farm income of W m when water is limited W ew. Therefore, in this paper, firstly the previous lengthy expressions for (W el) and (W ew) were simplified. Secondly, simple expressions for both (W el) and (W ew) were derived under presence of rainfall. Thirdly, very simple mathematical relations between (W m & W l) and (W m & W w) were derived in order to determine much easier expressions than those previously derived for W el , and W ew respectively, with and without rainfall.
DRAINMOD is a field and watershed scale water management simulation model that has been used to design subsurface drainage systems and characterize drainage and water table control practices in poorly drained soils. DRAINMOD is also used as a research tool to investigate the performance of drainage and sub irrigation systems and their effects on water use, crop response, land treatment of wastewater, and pollutants transport from agricultural fields. The model is based on a water balance in the profile to quantify hydrologic components such as infiltration, subsurface drainage, surface runoff, deep and lateral seepage and evapotranspiration. The objective of this research is to examine the capability of DRAINMOD to obtain an optimal drainage systems design for Iraqi conditions using the information of Dujailah project. This is done by obtaining the minimum drain depth and maximum spacing that meet the criteria for water table control. The results obtained from DRAINMOD for the drainage systems design show a good agreement with those previously obtained for Dujailah project. Therefore DRAINMOD is a suitable simulation model which can be used to simulate the performance of drainage and water table control systems in Iraq.
An irrigation scheduling model was developed based on daily soil water balance which takes into account the effect of water table on irrigation water requirement for wheat crop at Mosul area for eleven years. Capillary rise of water table was estimated with Darcy's equation .Crop evapotranspiration was estimated by pan evaporation method. The model inputs are daily climatological data for eleven years at Mosul Station, soil data (total available water, field capacity, permanent wilting point, allowable percent depletion, saturated hydraulic conductivity),and crop data(root depth for wheat crop, crop coefficient).The model estimates daily and seasonal actual evapotranspiration, effective rainfall, irrigation requirement and upward capillary rise from water table. Using the model for wheat, the seasonal irrigation requirements were reduced by 40% for silt loam soil and 90% for loam soil ,avoiding crop water stress or excessive irrigation.
A computer model was made to find drain spacing with Hooghout equation then SALTMOD was applied for water and salt balances of four different soils. Crop rotation with alfalfa in 50% of the land as continuous crop(in winter and summer) and 50% of the land wheat in winter then maize in summer season. Drain spacing can be increased than that calculated with Hooghout equation according to soil type and critical water table depth, So salinity, and water table depth were increased without exceeding root zone depth, in clay loam soil. Whereas it's increase has little effect on loamy sand soil, therefore, the best drain spacing can be chosen according to acceptable salinity in the soil. SALTMOD gives, depending on cumulative Gumbel distribution, a reasonable indication of the predicted change in soil salinity. Under the proposed conditions of irrigation and drainage after, 10 years , the root zone salinity in 80% of area irrigated with 0.5 ds/m water, was predicted to increase in summer to ,9.46, 8,7.53, 4.3 ds/m in the clay loam, loam, sandy loam, loamy sand soil respectively.
An irrigation scheduling model was developed based on daily soil water balance which takes into account the effect of water table on irrigation water requirement for wheat crop at Mosul area for eleven years. Capillary rise of water table was estimated with Darcy’s equation .Crop evapotranspiration was estimated by pan evaporation method. The model inputs are daily climatological data for eleven years at Mosul Station, soil data (total available water, field capacity, permanent wilting point, allowable percent depletion, saturated hydraulic conductivity),and crop data(root depth for wheat crop, crop coefficient).The model estimates daily and seasonal actual evapotranspiration, effective rainfall, irrigation requirement and upward capillary rise from water table. Using the model for wheat, the seasonal irrigation requirements were reduced by 40% for silt loam soil and 90% for loam soil ,avoiding crop water stress or excessive irrigation.
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