Project Title: Use of Remotely Sensed Data for Improved Quantification of Evapotranspiration for Water Management in Nebraska

Project PIs: A. Irmak, Derrel L. Martin, Suat Irmak, Shashi B. Verma, Donald Rundquist

Granting Agency Name:  UNL Water Center (funding from USGS)

Grant period:  01/2006-12/2008.

Poster presentation at Water Colloquium. October 27, 2006. Lincoln, Nebraska

Nature, Scope and Objectives of the Project, and Expected Outputs

We propose to develop scientific methodologies for quantifying ET at different scales, with emphasis on the major agroecosystems of the U.S. Corn Belt: rainfed and irrigated continuous maize and maize-soybean cropping. The overall goal of this project is spatiotemporal estimation of ET by utilizing satellite-derived spectral radiances in real time at watershed scale in Nebraska. The specific objectives are as following:

Objective 1: Estimate actual evapotranspiration and the biomass production under different agroecosystems in Nebraska using energy balance models with remote sensing data. Depending of adaptability of models to Nebraska region, either SEBAL or METRIC model will be used to quantify regional scale energy and water fluxes as derived from remote sensing inputs.

Expected outputs: The major outcome of this work would be quantitative surface energy balance algorithm for land estimates of ET for different cropping systems in Nebraska.

Objective 2: Assess the performance of the model results against several reference ground-truth ET techniques for testing its accuracy.

Expected outputs: Validation of energy balanced-based ET fluxes at field and landscape scales with different in situ measurement techniques. Seasonal variations of observed and estimated ET estimates will also be compared for determining an accurate baseline ET technique in the region.

Objective 3: Quantify ET over large areas at field and watershed scales in Nebraska. Identify and quantify uncertainties and errors that propagate through up- and downscaling processes.

Expected outputs: We will develop ET maps at different temporal scales across geographic space in Nebraska. Major outcome will be general guidelines, decision tools and algorithms estimating ET in soybean-corn based cropping systems to improve water resources management. These studies will enhance our understanding of what is required to more precisely estimate ET at scales that range from single fields to larger regions and to establish links between land use (different vegetation surfaces), water allocation, and water use.

Methods, Procedures, and Facilities

This project will utilize existing and ongoing measured ET data to assess the accuracy and adaptability of the SEBAL/METRIC for Nebraska soil, climate, and vegetation conditions. Good quality and large datasets of crop and reference ET are available at the IANR’s South Central Agricultural Laboratory (SCAL) near Clay Center, NE, and long-term Carbon Sequestration Project at the Agricultural Research Division (ARD) near Mead, NE. Spatial scales utilized for conducting measurements in this research range from points to small plots and the entire field. At the SCAL, a BREBS, ECS, ETgages (modified atmometers), and water balance have been used for continuous measurements of crop and reference ET over a corn crop (Figure 2). All these systems were installed in early 2004 in the same field. The field is 13.8 ha and irrigated with the subsurface drip irrigation. The research facility at the SCAL will give us a unique opportunity to assess the performance of the SEBAL against several reference ground-truth techniques for accuracy and performance in Nebraska.

The BREBS and ECS measure surface energy fluxes including solar and net radiation, latent heat, sensible heat, soil heat flux, air temperature, relative humidity, wind speed and direction, atmospheric pressure, and rainfall. Energy fluxes are being measured every 1/10 second with the ECS and every 30 seconds with the BREBS. The ETgage data are available every growing season on a daily basis. Soil water content is measured every 0.30 m up to 1.80 m in the soil profile all year long. In addition to these rich ET and soil water status measurements, crop and soil management, irrigation applications, crop growth and yield data are also available since early 2004. The ET measurements at the SCAL site are continuous and cover the measurements during the dormant (non-growing) season. This will provide an opportunity to assess the performance of the SEBAL to quantify evaporative losses during the winter months. Non-growing season evaporative losses have not been receiving enough attention by the researchers in Nebraska. However, preliminary data from BREBS and ECS at the SCAL show that significant evaporative losses occur during the winter months that will greatly affect the regional and field-scale water balances. The landsat 5 and Landsat 7 remotely sensed data will be used in SEBAL/METRIC methods.

Proposed Surface Energy Balance Methods

• Surface Energy Balance Algorithm for Land (SEBAL)

• Mapping Evapotranspiration at High Resolution and with Internalized Calibration (METRIC)

Surface Energy Balance with SEBAL/METRIC methods

R_n - G - λET - H + Δ = 0

where

Rn = Net radiation

G = Soil heat flux

λET = Latent heat flux

H = Sensible heat flux

Δ = Change in storage

Assumptions

• Principle of Energy Conservation

• Energy arriving at the surface = Energy leaving the surface

• Only vertical fluxes considered (Advection ignored)

• Minor energy components also ignored.

Methodology for SEBAL/METRIC methods