Remote sensing approach for the assessment and quantification of evapotranspiration

Field: Environmental Science (Physical Sciences)

Evapotranspiration (ET) plays a significant role in regional and global climate through impacting relationships between land-use/land cover change and microclimate/climate energy balance in the hydrological cycle and has important applications in agriculture and natural system. On regional scale, the application of satellite technology and remote sensing techniques in agricultural and hydrology has led to the development of surface energy balance algorithms [e.g. Surface Energy Balance Systems (SEBS)] that have been crucial to understanding and estimating surface energy fluxes e.g. ET and interactions over large spatial surfaces. The objective of this research were: (i) assess the performance of SEBS algorithms, using Landsat imagery, to estimate ETc and other energy fluxes with respect to BREBS-measured surface energy variables for several vegetation surfaces (Irrigated maize, winter wheat and rainfed grass) in the sub-humid and semi-arid transition zone in south central Nebraska, and (ii) quantify and evaluate the spatial and temporal distribution of ETc over south central Nebraska. We also investigate the effect of scale/ resolution on ET and other related fluxes using Landsat and Moderate Resolution Imaging Spectroradiometer (MODIS). The main objectives of this study were: (i) to analyze the ability of SEBS method for Landsat and MODIS satellite to estimate ETc over different vegetation surfaces in sub-humid/semi-arid transition zone in central NE and its validation with ground measurements, (ii) up-scaling/ aggregation of Landsat 30 m resolution ETc values by simple average, nearest neighbor to larger scales (60, 90, 120, 150, 240, 360, 480, 600, 750, and 1000 m), and (iii) to investigate the effect of aggregation on spatial distribution of energy balance fluxes and potential error associated with the aggregation process. The rapid expansion of the area under conservation tillage practices such as no-till (NT) and minimal tillage generally constitutes qualitative changes in cropping systems in the United States and other parts of the world. We use remote sensing tools to map the crop residue cover (CRC) over south central Nebraska. The main objectives of this study was (i) to evaluate the performance of different Landsat based tillage indices for estimating crop residue (ii) to map CRC in South Central Nebraska using multi-temporal Landsat imagery. Results from this research illustrate that the SEBS algorithms provide an important tool to estimate spatial distribution of surface energy balance variables over crop fields and for other large vegetation surfaces/areas within 10% margin of error as compared to the BREBS-measured values under these study conditions. Use of different satellites with varying resolution illustrate that the error propagation in flux retrieval with the increase in spatial resolution and difference in land-use characteristics. Land surface heterogeneity is essential to understand the scaling behavior and assessing the effect of the pixel resolution on the energy balance fluxes. This is the first study to map the CRC in Nebraska and among few studies in United States. Result showed that minimum Normalized Difference Tillage Index (NDTI) can be used to map the CRC. Result from this study illustrate that about 26% (158319 ha) of maize and 15% (42065 ha) of soybean land area in Adam, Clay, Fillmore, Hamilton, Kearney, Saline, Seward and York counties are having more than 70% CRC.