FROM INSTANTANEOUS TO AVERAGE DAILY EVAPORATIVE FRACTION, MINIMIZING UNCERTAINTIES, AND WATER BALANCE ESTIMATION OVER LARGE AREA USING COMBINED GEOSTATIONARY AND POLAR ORBITING SATELLITES. “CASE STUDY OF CASTILLA Y LEON REGION, SPAIN”

Water use strategy and water resources management requires scientifically sound information on water availability, particularly in the semi-arid areas like Castilla y Leon region, Spain (Fig.1). This region is suffering from high evaporation and low rainfall with negligible runoff. Due to this fact and climate change, the farming system of the region is converting from rainfed to irrigation. Therefore, actual evapotranspiration (AET) and rainfall are the main surface water balance components for this study area. Currently, both are potentially estimated from remote sensing observations. In the past decades, the quantification of daily actual evapotranspiration from remote sensing data was mainly based on one time observation from sun synchronous satellites by scaling instantaneous evaporation under the assumption of constant daytime evaporative fraction (EF). But, in this work, the average daily EF over large area was calculated using surface energy balance system (SEBS) model derived by Su (2002) with real time data from 40 ground meteorological stations and products from European organization for the exploitation of meteorological satellites on land surface analysis satellite application facility (EUMETSAT LSA SAF) and moderate resolution imaging spectroradiometer (MODIS) land surface products. The EUMETSAT LSA SAF products were derived from spinning enhanced visible and infrared imager (SEVIRI) radiometer embarked on the meteosat second generation (MSG) platform and other European satellite systems which has an imaging-repeat cycle of 15-30 minutes. The average daily EF was compared with the mid day EF (instantaneous EF) for different landcover classes (see Fig. 2) on 15^th of March, 2009. Finally, the simple water balance estimation of the region was carried out for the months of March, April, May and June, 2009 using rainfall products from EUMETSAT meteorological product extraction facility (MPEF) and daily AET. The results indicated that the average daily EF and instantaneous EF have shown strong relation (R²= 0.98, 0.90, 0.86, 0.73, 0.70 and 0.56 for water, irrigated croplands, rainfed croplands, shrubland, mosaic forest-shrubland and mosaic crop-vegetation respectively). But it shows poor agreement (R²=0.38, 0.32, 0.05 for Sparse vegetation, broadleaved deciduous forest and needleleaved evergreen forest respectively). In addition, the daily evapotranspiration was successfully validated with MSG evapotranspiration product (MET) and an eddy covariance system in sparse vegetation.

Key words; AET, SEBS, EF, EUMETSAT LSA SAF, MODIS, Water balance

Figure 1.The RGB (channel 1, 2 and 3) color composite of MSG images on July, 15, 2009 at 11:15 UTC covering the study area (left) and the drainage network of Catilla y Leon region (right).

Figure 2. Meteorological stations and landcover classes in the Catilla y Leon region

Acronyms

AET Actual Evapotranspiration

EF Evaporative Fraction

EUMETSAT LSA SAF European Organization for the Exploitation of Meteorological Satellites on Land

Surface Analysis Satellite Application Facility

MODIS Moderate Resolution Imaging Spectroradiometer

MPEF Meteorological Product Extraction Facility

MSG Meteosat Second Generation

SEBS Surface Energy Balance System

SEVIRI Spinning Enhanced Visible and Infrared Imager

Reference

Su, 2002. The Surface Energy Balance System (SEBS) for estimation of turbulent heat fluxes. Hydrology and Earth System Sciences, V. 6, no. 1: P.85-100.