|Agam, Nurit -|
Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 7, 2012
Publication Date: November 30, 2012
Citation: Colaizzi, P.D., Kustas, W.P., Anderson, M.C., Agam, N., Tolk, J.A., Evett, S.R., Howell, T.A., Gowda, P., O'Shaughnessy, S.A. 2012. Two-source energy balance model estimates of evapotranspiration using component and composite surface temperatures. Advances in Water Resources. 50:134-151. http://dx.doi.org/10.1016/j.advwatres.2012.06.004. Interpretive Summary: Irrigation of crops is important to maintain abundant food for a growing population. However, crop irrigation requires large amounts of water and energy. Both water and energy are becoming less available and more expensive. Therefore, it is important for farmers to conserve water and energy when irrigating crops. Conserving water and energy requires good irrigation management methods. One way to manage irrigation is by sensing crop temperature using infrared thermometers. The crop temperature is related to the rate of crop water use, which in turn is related to need for irrigation. When the crop does not completely cover the soil, which commonly occurs for row crops, the soil temperature beneath the crop will influence the crop temperature that is measured by the infrared thermometer. In order to get accurate estimates of the rate of crop water use, the influence of the soil temperature must be taken into account. We developed a new mathematical model that more accurately accounts for the influence of soil temperature on the infrared thermometer. This model was tested against actual measurements of crop water use, and the model greatly improved the accuracy of crop water use estimated with the infrared thermometer. This improved the usefulness of using crop temperature for irrigation management. This will help farmers continue to produce crops for a growing population, while using less water and energy.
Technical Abstract: The two-source energy balance model (TSEB) can estimate evaporation (E), transpiration (T), and evapotranspiration (ET) of vegetated surfaces, which has important applications in water resources management for irrigated crops. The TSEB requires soil (TS) and canopy (TC) surface temperatures to solve the energy budgets of these layers separately. Operationally, usually only composite surface temperature (TR) measurements are available at a single view angle. For surfaces with nonrandom spatial distribution of vegetation such as row crops, TR often includes both soil and vegetation, which may have vastly different temperatures. Therefore, TS and TC must be derived from a single TR measurement using simple linear mixing, where an initial estimate of TC is calculated, and the temperature – resistance network is solved iteratively until energy balance closure is reached. Two versions of the TSEB were evaluated, where a single TR measurement was used (TSEB-TR) and separate measurements of TS and TC were used (TSEB-TC-TS). Stationary infrared thermometers that viewed an irrigated cotton (Gossypium hirsutum L.) crop measured all surface temperatures (TS, TC, and TR). The TSEB-TR version used a Penman-Monteith approximation for TC, rather than the Priestley-Taylor-based formulation used in the original TSEB version, because this has been found to result in more accurate partitioning of E and T under conditions of strong advection. Calculations of E, T, and ET by both model versions were compared with measurements using microlysimeters, sap flow gauges, and large monolythic weighing lysimeters, respectively. The TSEB-TR version resulted in similar overall agreement with the TSEB-TC-TS version for calculated and measured E (RMSE = 0.7 mm d**1) and better overall agreement for T (RMSE = 0.9 vs. 1.9 mm d**1), and ET (RMSE = 0.6 vs. 1.1 mm d**1). The TSEB-TC-TS version calculated daily ET up to 1.6 mm d**1 (15%) less early in the season and up to 2.0 mm d**1 (44%) greater later in the season compared with lysimeter measurements. The TSEB-TR also calculated larger ET later in the season but only up to 1.4 mm d**1 (20%). ET underestimates by the TSEB-TC-TS version may have been related to limitations in measuring TC early in the season when the canopy was sparse. ET overestimates later in the season by both versions may have been related to a greater proportion of non-transpiring canopy elements (flowers, bolls, and senesced leaves) being out of the TC and TR measurement view.