Variation in the estimations of ET_o and crop water use due to the sensor accuracy of the meteorological variables

Abstract. In agricultural ecosystems the use of evapotranspiration (ET) to improve irrigation water management is generally widespread. Commonly, the crop ET (ET_c) is estimated by multiplying the reference crop evapotranspiration (ET_o) by a crop coefficient (K_c). Accurate estimation of ET_o is critical because it is the main factor affecting the calculation of crop water use and water management. The ET_o is generally estimated from recorded meteorological variables at reference weather stations. The main objective of this paper was assessing the effect of the uncertainty due to random noise in the sensors used for measurement of meteorological variables on the estimation of ET_o, crop ET and net irrigation requirements of grain corn and alfalfa in three irrigation districts of the middle Ebro River basin. Five scenarios were simulated, four of them individually considering each recorded meteorological variable (temperature, relative humidity, solar radiation and wind speed) and a fifth scenario combining together the uncertainty of all sensors. The uncertainty in relative humidity for irrigation districts Riegos del Alto Aragón (RAA) and Bardenas (BAR), and temperature for irrigation district Canal de Aragón y Cataluña (CAC), were the two most important factors affecting the estimation of ET_o, corn ET (ET_{c_corn}), alfalfa ET (ET_{c_alf}), net corn irrigation water requirements (IRn_corn) and net alfalfa irrigation water requirements (IRn_alf). Nevertheless, this effect was never greater than ±0.5% over annual scale time. The wind speed variable (Scenario 3) was the third variable more influential in the fluctuations (±) of evapotranspiration, followed by solar radiation. Considering the accuracy for all sensors over annual scale time, the variation was about ±1% of ET_o, ET_{c_corn}, ET_{c_alf}, IRn_corn, and IRn_alf. The fluctuations of evapotranspiration were higher at shorter time scale. ET_o daily fluctuation remained lower than 5 % during the growing season of corn and alfalfa. This estimation fluctuation in ET_o, ET_{c_corn}, ET_{c_alf} , IRn_corn, and IRn_alf at daily time scale was within an acceptable range, and it can be considered that the sensor accuracy of the meteorological variables is not significant in the estimation of ET_o.