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, the daily soil water depletion should be estimated during the crop growth period. Soil water depletion is dependent on daily evapotranspiration. In this research, daily evapotranspiration of S.C 704 maize was measured in mini-lysimeters. Estimation of daily evapotranspiration was done by continuous measurement of soil moisture. Leaves stomatal resistance was measured daily, by AP4 Porometer device. Soil water allowable depletion was determined in four growth stages of initial (C1), development (C2), mid (C3), and late (C4), based on the leaves stomatal resistance response. At each growth stage, when leaves stomatal resistance increased relative to the control crops, readily available water was ending and the time was right for new irrigation. The main variables included growth stage effect on crop evapotranspiration and water depletion coefficient, which was investigated in a completely randomized basic design, with three replications. Regression functions (models) were used for simulation of allowable soil water depletion coefficient (P) based on the daily evapotranspiration (ETc). The models were calibrated by daily data at initial and development stages, and were evaluated by daily data in mid and late stages. The FAO-56 linear model was compared with the models introduced in this research. The results showed that maize ETc (S.C 704) 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 in the mentioned stages was calculated as 0.45, 0.66, 0.61 and 0.7, respectively. Different sensitivity in crop growth stages caused readily available water limit not to be constant during growth period. The ETc increase caused a decrease in P, and decrease in ETc increased P. 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=-0/000005, R2=0.999 and EF=0.999, was the optimal model in estimation of P coefficient. The reason for weak performance of FAO-56 model was the constant limit for readily available water and mean ETc rate in the 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, especially those with short intervals.
Keywords
)