Document Type : Research Paper
Author
Department of Water Engineering, Faculty of Agriculture and Natural Resources, Imam Khomeini International University, Qazvin, Iran.
Abstract
For an accurate irrigation schedule, should be estimated the daily soil water depletion amount during the crop growth period. Soil water depletion is dependent to daily evapotranspiration amount. In this research, daily evapotranspiration of S.C 704 maize was measured in mini-lysimeter. Estimation of daily evapotranspiration was done by continuous measurement of soil moisture. Leaves stomatal resistance was measured as daily, and by AP4 Porometer device. Soil water allowable depletion limit in four growth stages of initial(C1), development(C2), mid(C3) and late(C4), was determined based on the leaves stomatal resistance response. The time of leaves stomatal resistance increasing (at each growth stage) relative to control crops, was the time of readily available water ending and doing of new irrigation. Main factor was included of growth stage effect on crop evapotranspiration and water depletion coefficient amounts, which was investigated in a completely randomized basic design, with three replications. Regression functions (models) were used for simulation of soil water allowable depletion coefficient (P) based on the daily evapotranspiration (ETc). The models were calibrated by daily data in initial and development stages, and they were evaluated by daily data in mid and late stages. The FAO-56 linear model was compared with introduced models in this research. The results showed that maize ETc (S.C 704) rate in initial, development, mid, and late stages was in the range of 1.5-4.5, 3.9-7.1, 1.4-7.5, and 0.2-2.1 mm.d-1, respectively. The allowable soil water depletion limit in mentioned stages was calculated as 0.45, 0.66, 0.61 and 0.7, respectively. Different sensitivity in crop growth stages was caused that, readily available water limit not be constant in growth period. The ETc increasing was caused a decreasing in P coefficient, and ETc decreasing was caused an increasing in P coefficient, in growth stages. Linear, exponential, logarithmic, polynomial, power, and FAO-56 linear functions, were investigated. Polynomial function with statistical indices of RMSE=0.00035, NRMSE=0.054, ME=0.0008, CRM=-000005, R2=0.999 and EF=0.999, was the optimal model in estimation of P coefficient. The reason of weak performance in FAO-56 model, was considering the constant limit for readily available water and mean ETc rate in growing season. Therefore, the FAO-56 model was modified. The research result was to estimate the soil water allowable depletion coefficient (by using ETc), without daily measurement of soil moisture. This method will be useful in irrigation scheduling with short rounds, especially. So that, the suitable time and volume of irrigation are determined for water loss prevention.
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