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United States Department of Agriculture

Agricultural Research Service

Research Project: Crop Evapotranspiration Determination Using Eddy Covariance Fluxes, High Resolution Remote Sensing Imagery & a Surface Temperature Approach

Location: Soil and Water Management Research

2013 Annual Report

1a.Objectives (from AD-416):
1. Evaluate eddy covariance (EC) latent (LE) and sensible (H) heat fluxes using large monolithic weighing lysimeters' data; 2. Develop and evaluate algorithms to estimate surface aerodynamic temperature (SAT) and aerodynamic surface roughness lengths [for momentum (Zom) and heat transfer (Zoh)] using wind/temperature profiles, high resolution remote sensing (RS) imagery, and EC data; 3. Evaluate the surface renewal (SR) model for its capability to estimate crop and native grassland actual ET using wind/temperature profiles, scintillometer, and EC data; 4. Develop and evaluate a RS-SAT-based single source energy balance (EB) model, to accurately estimate spatially distributed ET, using measured evapotranspiration (ET) by large monolithic weighing lysimeters and a network of EC EB systems; and 5. Evaluate a modified RS-based two-source EB model (M-TSM) using large monolithic weighing lysimeters.

1b.Approach (from AD-416):
This agreement refers to activities involving the processing and analysis of data collected during the Bushland Evapotranspiration and Agricultural Remote Sensing Experiment of 2008 (BEAREX08) over an area within the limits of the USDA-ARS, Conservation and Production Research Laboratory (CPRL), at Bushland, Texas. The CPRL is located 1170 m above mean sea level at 35 degrees 11' N, 102 degrees 06' W, in the Texas High Plains. Objective 1: The EC raw data (i.e., times series high frequency data) will be processed using the micrometeorological applications software EdiRe. Different levels of data processing will be performed and resulting LE and H values will be compared to lysimetric data. Objective 2: In mapping ET, LE can be spatially estimated as an EB residual for land surfaces using remote sensing inputs. The EB equation requires the estimation of net radiation (Rn), soil heat flux (G), and H. Rn and G can be estimated with acceptable accuracy. However, H may be under estimated when the radiometric surface temperature (RST) is used rather than the surface aerodynamic temperature (SAT) in the aerodynamic resistance equation. The objective is to model SAT to improve the estimation of H and consequently ET for the semi-arid, advective environment of the Texas High Plains. Objective 3: High frequency (1 Hz) wind, air temperature and relative humidity data (profile) recorded at six heights, over a native grassland/rangeland during the fall of 2007 and winter of 2008, will be used in surface renewal (SR) analysis in order to estimate H. Estimates of H will be compared to measured H by a large aperture scintillometer (LAS). H will be computed using different combinations of heights (i.e., of U and Ta recorded at given heights). Similarly, for a dryland cotton crop (2008 data), H will be estimated using the SR approach; however, resulting H values will be compared to measured H by an EC system. Objective 4: Instantaneous, daily, weekly, monthly, and seasonal estimates of spatially distributed ET will be mapped using very high resolution airborne surface reflectance and radiometric surface temperature images along with weather data in the newly developed SAT-based single source EB model. Distributed estimates of Rn, G, and H will be compared to measured values. In the case of H (as for LE) a footprint analysis will be used. Objective 5: Instantaneous and daily ET maps will be produced using a modified two-source EB model (M-TSM), airborne-based surface reflectance/temperature images, and ground-based agroclimatological data. Data obtained during the 2007 BEAREX campaigned will be used in the parameterization process and data collected during BEAREX08 will be used in the application and verification of the M-TSM. Model verification will be performed using data from the large weighing lysimeters.

3.Progress Report:

Regarding objective 1, an article entitled "Eddy covariance heat flux processing for ET estimates in an advective environment" is being re-worked for submission to Irrigation Science. A second article, "Validation and adjustment of eddy covariance ET considering heat flux source area" is being internally reviewed and formatted for submission to Irrigation Science as well. With regard to objective 4, the surface aerodynamic temperature (SAT) algorithm for the estimation of cotton ET (COSAT) is still under development. A procedure to compute sensible (H) and latent (LE) heat fluxes from data collected with the "Aerodynamic Profile Tower" or APT system was defined and tower data from 2008 were processed to calculate the sensible (H) and latent (LE) heat fluxes. The procedure was applied to the 2008 BEAREX Bushland data (southwest SW lysimeter field), and heat fluxes were determined (spreadsheet with processed data is available under request, size 119.5 MB). The next step will be to evaluate the APT H and LE fluxes using data from an eddy covariance energy balance station (site.
5)co-located with the APT system. Thereafter, the next step will be to calibrate a COtton Surface Aerodynamic Temperature (COSAT) model to map cotton ET spatially. The COSAT model will be applied to 2008 airborne remote sensing data, and ET maps will be evaluated with the network of EC systems and lysimeters' ET data. For this purpose, the 2008 processed imagery (surface reflectance and temperature) collected under the BEAREX08 campaigns is now requested from the USDA ARS CPRL. The PI does not have these data available at this moment. For this reporting period, the progress towards executing objectives 1 and 4 has been limited due to unavailability of a suitable graduate student and due to the PI's increased work load. For the coming reporting period (2013-2014) a suitable graduate student has been identified. The student has started working in the project. Preliminary results (Mkhwanazi and Chavez, 2012) on using the surface aerodynamic temperature approach to map ET were presented at the Tri-Society meeting in Ohio, October 2012. Results indicated that using measured surface aerodynamic temperature (To) in the sensible heat flux sub-model (energy balance) produced an ET rate that was more accurate and closer to ET values measured with a Bowen ratio energy balance system, operated by the USDA ARS, than ET values obtained with other methods. These results are encouraging in that they are evidence that the To approach is a valid approach to map ET accurately. The paper "Using surface aerodynamic temperature in remote sensing-based ET models (Aerodynamic Surface Temperature Modeling Under Various Surface Conditions)" was presented at the 2012 Annual International ASA-CSSA-SSSA Conference Meeting. Symposium-Evapotranspiration: Monitoring, Modeling and Mapping At Point, Field, and Regional Scales. Cincinnati, Ohio, 23 Oct. APT method limitations Some limitations for the profile method, in calculating H and LE, were identified: 1. Identifying correctly the true atmospheric stability condition for the entire profile tower for each time step/stamp. Uncertainty occurs because there are two different criteria (Zm vs. Ri or Zm vs. Ri/(1-5Ri)) for stable and unstable atmospheric conditions and sometimes in some levels different conditions were found. The stability can only be known after knowing the sign of the Monin-Obukhov length and/or by the difference in temperature at two consecutive heights. However, the overall Monin-Obukhov stability length is obtained only after plotting Zm vs. Ri or Zm vs. Ri/(1-5Ri). 2. Several data points were filtered out since the footprint/fetch limit was not satisfied in many instances. A larger field (dimensions) would be needed to be able to use all levels of the APT system for the conditions encountered in the Texas High Plains. Conclusions: The surface aerodynamic temperature approach to map ET has shown promise, which motivates further investigation into the data acquired in the BEAREX07 and BEAREX08 evapotranspiration field campaigns at Bushland. ARS will supply the requested 2008 processed imagery for surface reflectance and temperature.

Last Modified: 7/28/2014
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