Evaluación de la evapotranspiración de productos satelitales en la zona sur del ecuador, 1990-2013

Abstract

Over the years, the indiscriminate use of fresh water has diminished the planet's natural reserves and conventional agriculture is a participant in this abuse. In order to control this fact to some extent, calculations of water needs are used and the most popular method is the calculation of evapotranspiration (ETo), which allows controlling irrigation and conserving water for agriculture. The Hargreaves method allows estimating ETo, using air temperature and extraterrestrial radiation. In general, the Hargreaves equation is recommended for weekly calculations or even for much longer periods, although there have been cases in which very accurate daily ETo estimates have been found. Satellite images are obtained through numerous remote sensors, there is a sensor that is used very frequently which is MODIS, it allows estimating from various inputs such as: the type of soil cover, leaf area index, albedo, among others. The general objective of the research is to evaluate the evapotranspiration of satellite products in the southern zone of Ecuador, period 1990-2013, using Hargreaves (HG) as a reference for the estimation of correlation and efficiency of remote sensing information. The research included the study of 15 meteorological stations for ETo determination by means of Hargreaves and MODIS, which are located in the southern part of Ecuador. Geographically, it is located between the parallels of 3º and 5º south latitude and the meridians of 78º and 81º west longitude. The southern zone of Ecuador includes the Jambelí Archipelago, the coastal zone of the province of El Oro, the highlands passing through the province of Loja and the Amazonian part belonging to the province of Zamora Chinchipe. Information was taken from the yearbooks of the official website of INAMHI (National Institute of Meteorology and Hydrology), the monthly scale data were digitized in the time series from 1990 to 2013, where variables of maximum and minimum temperature were considered, from which the average temperature was derived as a necessary variable for the calculation of evapotranspiration. Data extraction was performed using MODISTools, NDVI and EVI were downloaded at 500m2 resolution and with a good pixel reliability to allow quality control before analysis. The Hargreaves (HG) equation was used to calculate ETo, a method based on the use of temperature and solar radiation. The exploratory analysis of the climatic elements of the meteorological stations was carried out through a study where 15 stations were analyzed, 4 of which are located in the province of El Oro, 10 in the province of Loja and the remaining one in the province of Zamora Chinchipe. This was achieved through the description of graphics generated in ArcMap where a dividing line is denoted which corresponds to the Andean zone of Ecuador, which is above 1000 meters above sea level where 62.5% of the stations are distributed in this territory. Potential evapotranspiration was estimated by Hargreaves on an annual basis where scatter plots were grouped by provinces for a better understanding of the behavior of ETo values over the years, where there were significant fluctuations and gaps in values. The correlation between ETo from the Hargreaves equation and MODIS showed a weak positive Pearson correlation coefficient with a value of 0.31, the p-value of the analysis of variance is 0.001; consequently, there is a significant relationship between the variables with a confidence level of 95%. Furthermore, the coefficient of determination or goodness of fit only explains 10 of the variability between HG and MODIS, implying a weak relationship between the variables. The RMSE value indicates that the standard deviation of ETo is 1.01 mm/day, also a Bias or underestimated bias of -11.52 is obtained.