A comparative study of ground and satellite evapotranspiration models for southern California

This project offers a comparative study between a ground-based weather station and satellite-based method of calculating evapotranspiration (ET). Using four selected cloudless days measured between 10:28 am and 11:30 am DST local in Los Angeles in 2016 (08 February, 19 April, 22 June, and 08 July), the aim of this project is to determine if the Normalized Difference Vegetation Index (NDVI) Triangle method, a satellite-based methodology, could be used as an acceptable alternative to measure ET for locations without a ground-based weather station. ❧ The Moulton Niguel Water District (MNWD), located in Orange County, California was selected as the study area for this project. Weather station #245, located on the Coto De Caza golf course, generated the ground-based weather data for the Penman-Monteith calculation of evapotranspiration for the MNWD. In contrast, the satellite method used the Landsat Data Continuity Mission’s (LDCM) multi-spectral and thermal bands to calculate ET by Land Surface Temperature (LST) and NDVI values. These two data inputs, by the application of band math, were combined to create a two-dimensional NDVI Triangle pixel plot evapotranspiration estimate for each day calculated. The data from the two methodologies were then compared, with the assumption the Penman-Monteith method was the most accurate measure of ET. The results were statistically compared and analyzed for accuracy through Root Mean Square Error (RMSE), Mean Absolute Deviation (MAD), Mean Absolute Percentage Error (MAPE) and the coefficient of determination calculations. The results, although limited by sample size, indicate the NDVI Triangle method can be used to estimate ET. The process of creating the NDVI Triangle 2-D plot from multi-spectral and thermal bands from the LDCM is presented in detail, in addition to the process of defining the dry and wet edges of the plotted NDVI Triangle.本研究针对地面气象站法与卫星反演蒸散发（evapotranspiration, ET）的两种方法开展对比分析。本研究选取2016年洛杉矶当地夏令时10:28至11:30时段内的4个无云观测日期（分别为2月8日、4月19日、6月22日及7月8日），旨在验证基于卫星的归一化植被指数三角（NDVI Triangle）法能否作为无地面气象站区域蒸散发量的可靠替代测量方案。 本研究的研究区域选定为加利福尼亚州奥兰治县的莫尔顿·尼格尔水区（Moulton Niguel Water District, MNWD）。位于科托德卡扎高尔夫球场的245号气象站，为该区域蒸散发量的彭曼-蒙特斯（Penman-Monteith）法计算提供了地面气象观测数据。与之相对，卫星反演方法则借助陆地卫星数据持续任务（Landsat Data Continuity Mission, LDCM）的多光谱与热红外波段，基于地表温度（Land Surface Temperature, LST）及归一化植被指数（NDVI）值计算蒸散发量。通过波段运算将这两类输入数据进行融合，可针对每日的计算结果生成二维NDVI三角像素图，进而实现蒸散发量的估算。随后对两种方法得到的数据开展对比分析，本研究假设彭曼-蒙特斯法为蒸散发量的最精准测量手段。通过均方根误差（Root Mean Square Error, RMSE）、平均绝对偏差（Mean Absolute Deviation, MAD）、平均绝对百分比误差（Mean Absolute Percentage Error, MAPE）及决定系数的计算，对两种方法的结果开展统计学对比与精度分析。尽管受样本量限制，研究结果仍表明NDVI三角法可用于蒸散发量的估算。本研究不仅详细阐述了基于LDCM多光谱与热红外波段生成二维NDVI三角图的完整流程，还明确了如何划定该NDVI三角图的干、湿边界。