Submitted to: Decennial National Irrigation Symposium
Publication Type: Proceedings
Publication Acceptance Date: 12/2/2010
Publication Date: 12/5/2010
Citation: Evett, S.R., Lascano, R.J., Howell, T.A., Tolk, J.A., Oshaughnessy, S.A., Colaizzi, P.D. 2010. Single-and dual-surface implicit energy balance solutions for reference ET. Decennial National Irrigation Symposium, December 5-8, 2010, Phoenix, Arizona. Paper no: IRR10-8347.
Interpretive Summary: Farmers aim to schedule crop irrigations so that crop water needs are met without over irrigating. Over irrigation may lead to water logging and often results in loss of valuable nitrogen fertilizers. Nitrogen lost due to over irrigation can reach the aquifer or streams causing nitrate levels exceeding safe drinking water standards or causing algal blooms in lakes and the ocean. Because of the irrigation scheduling need, state and federal agencies have developed weather station networks in the most heavily irrigated western states so that the weather data can be used to estimate crop water use. The equations used to estimate water use from weather data are not completely accurate, however. So, scientists of the USDA-ARS Conservation & Production and the Cropping Systems Research Laboratories collaborated to assess possibly more accurate ways to estimate water use. Two methods were developed and tested against both the measured water use and the water use estimated using the current best method. All three methods estimated crop water use well, with one of the newly developed methods showing some improvement over the other two. More importantly, a crop-related parameter used in all three methods was shown to be inaccurate in some weather circumstances. Recommendations were made for improving the values assigned to that parameter.
Technical Abstract: The concept of a reference evapotranspiration (ETr) calculated from daily or hourly weather data, multiplied by a crop coefficient (Kc), in order to estimate crop water use (ETc), is widely established in agricultural science and engineering. To find region and variety-specific values of Kc from field-measured ETc values, the equation is inverted to: Kc = ETc/ETr. Forms of the Penman-Monteith (PM) formula for calculation of reference alfalfa or grass evapotranpsiration (ETr and ETo, respectively), have been promulgated by ASCE in 1990, FAO in 1998, and ASCE in 2005. The PM formulations are sensitive to climatic conditions, producing estimates of ETr and ETo that are more or less close to measured values depending on regional climate, and yielding values of Kc that vary from region to region and so are not transferrable. Theoretical shortcomings may be the basis of some of these problems, including the explicit nature of the calculation, which relies on the implied assumption that canopy and air temperatures are equal. We tested two surface energy balance formulations that stipulated different air and canopy temperatures, one a two-layer (soil and canopy) and one a one-layer (big leaf) approach but with soil heat flux included. Since canopy temperature is implicit in these formulations, they must be solved iteratively. Iterative solutions of ETr were compared with the FAO and ASCE PM formulations and against lysimeter-measured ETr. All three methods of ETr estimation produced ET values that compared very well with field-measured ET for alfalfa grown under reference ET conditions. Errors may occur with any of the three approaches to ETr estimation when stomatal resistance changes due to weather conditions; and assumptions of constant daytime and nighttime surface resistances thus cause mis-estimation of surface energy fluxes. It appears that a surface resistance value of 200 s m**-1 at night for alfalfa grown under reference ET conditions is too large. It also appears that assuming constant daytime surface resistance of 45 s m**-1 is probably not ideal, and that presenting daytime surface resistance as a function of vapor pressure deficit might improve ETr prediction.