Spatial and Temporal Properties of Reference Evapotranspiration and Its Related Climatic Parameters in the Main Agricultural Regions of Iran.

Variation in climatic parameters affecting water access can play a key role in the sustainable development of agriculture and the environment. Reference evapotranspiration (ET₀), after precipitation, is the most important parameter affecting access to water resources in the arid and semiarid areas like Iran. In this study, data were collected from 50 synoptic weather stations for a period of 30 years (1988–2017) in the area with high population density and critical agricultural regions of Iran. Trend analysis of ET₀ and climatic parameters including maximum and minimum temperature (T_max and T_min), sunshine duration (SD), relative humidity (RH), wind speed (WS), and air pressure (P) were investigated. Also, the effects of climatic parameters on ET₀ were analyzed using dimensionless sensitivity coefficients. The aridity index (AI) was also evaluated using precipitation and ET₀ data in the study area. Results showed that on an annual time scale, ET₀ had a significant increasing trend (p ≤ 0.05) in 77.1% of the studied stations. In the seasonal time scale, the most increasing trend of ET₀ was observed in winter. The results of sensitivity analysis showed that T_max, RH, and WS were the most effective variables on ET₀. The trend analysis of AI revealed that 92% of the stations had a significant (p ≤ 0.05) increasing trend over the study period. The spatial distribution of the annual AI indicated that only 10% of the study area meets the atmospheric evaporation demand, and in 90% of the study area, the annual ET₀ was greater than the annual precipitation. These areas were highly affected by aridity and thus required additional water for crop production. These results indicated that climate change in Iran has increased ET₀ and aridity, thereby increasing the water requirement for plants in agricultural lands. Hence, the optimum use of water resources for agricultural purposes is a suitable way to deal with the crisis of water shortage and aridity in Iran. [ABSTRACT FROM AUTHOR]