Abstract
The Surface Energy Balance Algorithm for Land (SEBAL) is one of the remote sensing (RS) models that are increasingly being used to determine evapotranspiration (ET). SEBAL is a widely used model, mainly due to the fact that it requires minimum weather data, and also no prior knowledge of surface characteristics is needed. However, it has been observed that it underestimates ET under advective conditions due to its disregard of advection as another source of energy available for evaporation. A modified SEBAL model was therefore developed in this study. An advection component, which is absent in the original SEBAL, was introduced such that the energy available for evapotranspiration was a sum of net radiation and advected heat energy. The improved SEBAL model was termed SEBAL-Advection or SEBAL-A. An important aspect of the improved model is the estimation of advected energy using minimal weather data. While other RS models would require hourly weather data to be able to account for advection (e.g., METRIC), SEBAL-A only requires daily averages of limited weather data, making it appropriate even in areas where weather data at short time steps may not be available. In this study, firstly, the original SEBAL model was evaluated under advective and non-advective conditions near Rocky Ford in southeastern Colorado, a semi-arid area where afternoon advection is common occurrence. The SEBAL model was found to incur large errors when there was advection (which was indicated by higher wind speed and warm and dry air). SEBAL-A was then developed and validated in the same area under standard surface conditions, which were described as healthy alfalfa with height of 40–60 cm, without water-stress. ET values estimated using the original and modified SEBAL were compared to large weighing lysimeter-measured ET values. When the SEBAL ET was compared to SEBAL-A ET values, the latter showed improved performance, with the ET Mean Bias Error (MBE) reduced from −17.1% for original SEBAL to 2.2% for SEBAL-A and the Root Mean Square Error (RMSE) reduced from 25.1% to 10.9%, respectively. It was therefore concluded that the developed SEBAL-A model was capable of accounting for advection and therefore suitable for arid and semi-arid regions where advection is common.
Highlights
Remote sensing is becoming an important technique in the estimation of crop evapotranspiration (ET)
While the original Surface Energy Balance Algorithm for Land (SEBAL) model seems appropriate in estimating hourly ET (NSCE = 0.5), during the time of satellite overpass, it was found to be unsuitable for estimating daily ET
From the results obtained from the study, it can be concluded that the SEBAL model usually underestimates ET mostly due to advection, with bias errors of up to 40% being observed for alfalfa in southeastern Colorado, U.S The daily ET estimation resulted in larger errors than for hourly ET when
Summary
Remote sensing is becoming an important technique in the estimation of crop evapotranspiration (ET). ET is a component of the hydrologic cycle, and its accurate estimation is of significant importance in the improvement of agricultural water management. Several methods have been used to directly and indirectly measure or estimate ET, and these include weighing lysimeters, using reference ET and tabulated crop coefficients, Eddy Covariance method (EC), the Bowen Ratio Surface Energy Balance method (BREB), scintillometry, atmometers, soil water content and tension sensors, and lately remote sensing models. Remote sensing methods have the advantage of having the capability to cover larger areas and are, suitable for regional analysis. With reasonable accuracy in ET estimation, the remote sensing-based approach promises to be a critical tool in improving crop water management. Several remote sensing models have been developed.
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