Title: Sensitivity Analysis of FAO-56 Penman-Monteith Reference Crop Evapotranspiration Estimation Method to Changes in Meteorological Inputs in Greenhouses
Authors: Amna M.A. Gaafer
Hassan I. Mohammed
Abdelkarim D. Elfadil
Volume: 7
Issue: 12
Pages: 30-37
Publication Date: 2023/12/28
Abstract:
Estimation of reference evapotranspiration (ETo) using FAO-56 Penman-Monteith method (PM) in greenhouses is critically needed for irrigation scheduling, water management and design of their irrigation system. Such estimation is confronted, in many cases, by unavailability of adequate and precise climatic input data. Hence, this study analyzes the sensitivity of estimating PM - reference evapotranspiration (ETo) to climatic variables in greenhouses with the objective of minimizing the effort in their precise collection without signi?cant loss of information. Therefore, it is assumed that assessing the FAO guidelines to compute ETo when meteorological data are missing could lead to a better understanding of which variables are critically important for reliable estimates of ETo. The needed meteorological inputs in greenhouses on daily basis are the maximum (Tmax), minimum (Tmin) air temperature, solar radiation (Rs), average relative humidity (RH avg), and wind speed (U2 at 2 m height). However, Rs may be predicted from temperature data. Sensitivity analysis in this study is performed by changing (increasing and decreasing) each one of these climate variables by one unit of (10% increment and decrement) for ten cases. Inside house climate data was measured from nine greenhouses in three areas with three houses per areas around Khartoum-Sudan (El Alafoon, Halfaya, and Shambat) for a period of three months in each house. Data where taken at ten day per month (every other three days) at three times per day (morning, mid-noon and evening). Sensitivity of each climate variable to predict ETo was assessed using descriptive statistics (standard deviation - std, coefficient - CV%, and t-test), regression coefficient - r2, slope and Christiansen uniformity of distribution (Ed). The results showed that the change in (ETo) is linearly related to change in all climate variables (r2 = 0.94) except wind speed (U) (r2 = 0.46 to 0.68) at all years. Further, According to relative dispersion (CV %), Std. and t-test ETo is significantly sensitive to (Tmax), (Tmin) followed by (RH) and least sensitive to (U) at all years. This result imply that determination of ETo inside greenhouse require more accuracy in determining Tmax, Tmin, RH than Rs and U2 which can be estimated with reduced precision.