Evaluation of Penman-Monteith estimates of evapotranspiration derived using field-collected stomatal conductance observations

Accurate estimation of evapotranspiration (ET) is critical for various applications in hydrology and agricultural water management. However, direct observations of ET, specially its spatial variation, in time-consuming and cumbersome, thus necessitating the need to use of indirect methods for its estimation. In this study, stomatal conductance data is used in conjunction with bio-physical parameters of wheat crops for deriving the spatially varied estimates of ET (ET_SC) for different irrigation treatments using the Penman-Monteith equation. For this, five treatments, including drip (DI) and flood (FI) irrigated treatments were used in the study, namely fully irrigated (DI)), 50% MAD (maximum allowable deficit) (DI), 50% MAD (FI), farmer fields replication (FI) and rain-fed treatment.

The ET_SC estimates are also compared to the ET estimates derived using a method based on field water balance (ET_WB). When compared with the ET_WB values, the ET_SC estimates compared well particularly for the irrigated treatments. The average root mean square error (RMSE) of ET_SC estimates in comparison to ET_WB values are 0.11, 0.2, 0.23 and 0.26 mm/day for fully irrigated, 50% MAD (FI), 50% MAD (DI) and farmers field replication treatments, respectively. The corresponding RMSE value (0.47 mm/day) for the rain-fed treatment are found significantly higher than the irrigated treatments indicating the limitation of the approach in high water stress conditions. The differences between ET_SC andET_WB values also increase significantly during the end-season stage when the wheat crop is close to maturity. Overall, the results demonstrate the robustness of the proposed approach in estimating the spatial variation of ET using the Penman-Monteith method in conjunction with the on-field field stomatal conductance observations.