Assessing the Reliability of Satellite-Derived Evapotranspiration Data Using Numerical Modified Penman Method at Citarum Watershed

Evapotranspiration (ET) is crucial in the analysis and estimation of water availability and demand for crops. Due to limited discharge measurement stations, assessing water availability becomes a challenge in water management planning. Specifically, Citarum watershed, the biggest watershed in West Java, serves as a primary source of raw water for Jakarta, Indonesia’s capital city. Modified Penman method, which was originally developed by the Food and Agriculture Organization (FAO) and subsequently modified for tropical areas, is commonly employed to analyze ET. In water balance equation, ET represents one of its key components, and solving this equation becomes necessary to determine water losses accurately. Accordingly, the standard product of the Moderate Resolution Imaging Spectroradiometer (MODIS) satellite, MOD16A2, offers an alternative source of ET data. To address the absence of ground station data, the reliability of satellite data becomes crucial. Therefore, the aim of this study is to compare and analyze the reliability of satellite-derived ET potential images with numerical Modified Penman method specifically within Citarum Watershed. This method is one among several other methods used to calculate ET potential based on climate data. The MOD16A2 and Modified Penman equation were used for the simulation and baseline data respectively. The reliability of the two simulations was analyzed by examining the skewness percentage of each pixel and period. By observing the distribution of percent skewness, it becomes possible to evaluate the performance comparison of two simulations. Lastly, it is important to note that the sensitivity of satellite is significantly affected by local weather conditions.