Investigation of impact of plant water requirement calculation method, length of maximum water requirement period and probability level on optimal estimation of crop water requirement (Case Study: Hamedan)

The recent research es indicate that irrigating with mean values of crop reference evapotranspiration (ETo) level will only provide about 50% of plant water demand. In such cases, irrigation level may not provide optimum water for plant and plant may face with water stress or water excess. With respect to the impact of ETo estimation methods, irrigation schedule period, and optimal probability level on plant water requirement, accurate evaluation of each case seems necessary to be studied. Using climatologies (1977-2006) of daily meteorological data, the daily ETo is estimated by means of five commonly used evapotranspiration methods (Blaney-Criddle, Penman, Penman-FAO24, Penman-Montieth and Penman-Montieth FAO-56). To evaluate the influence of each ETo models on actual plant water requirement, the daily evapotranspiration values were determined for wheat (ETc) during the period of study (1977-2006). The dates of starting and ending of maximum water requirement (MWR) were determined by means of moving average approach. Using the extracted dates, mean daily ETo values were plotted for different periods from one day to 30-days of MWR and different probability levels. The results indicated that if we apply the mean daily ETc values in designing irrigation systems (7-day period), at 50% probability level and Blaney-Criddle, Penman-Montieth FAO-56, Penman, Penman-FAO24 and Penman-Montieth ET0 methods, the water capacity of irrigation system will respectively be 12%, 10%, 18%, 14%, and 15% less than the corresponding values of 75% probability level. The statistical analysis of daily (ETc) data also showed that by increasing the MWR period, the estimated mean daily evapotranspiration would decrease. In addition, for a specific period, using higher probability level causes more crop water requirement values.