Quality assurance of weather parameters for determining daily evapotranspiration in the humid growing environment of the Mid-South

Authors: Sassenrath, Gretchen; Schneider, Jeanne; SCHMIDT, AMY - Mississippi State University; SILVA, A - Mississippi State University

Submitted to: Journal of the Mississippi Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/16/2012
Publication Date: 6/1/2012
Citation: Sassenrath, G.F., Schneider, J.M., Schmidt, A.M., Silva, A.M. 2012. Quality assurance of weather parameters for determining daily evapotranspiration in the humid growing environment of the Mid-South. Journal of the Mississippi Academy of Sciences. 57:178-192.

Interpretive Summary: Crop models and decision support tools depend on inputs such as weather data and soil parameters to determine various aspects of crop development for scheduling management inputs. These decision support tools and simulations are only as accurate as the data they depend on. Crop producers in humid regions have increasingly relied on irrigation to enhance productivity and insure return on investment. A network of weather stations has been developed in Mississippi, primarily in the heavily agricultural area of the Mississippi River Alluvial Flood Plain, colloquially known as the Delta. The series of weather stations is maintained by the Delta Agricultural Weather Center and the National Resources Conservation Service, and the data is publically available on their web sites. Researchers at USDA Agricultural Research Service and Mississippi State University have developed an irrigation scheduling tool that uses weather data to calculate reference evapotranspiration (ET). The reference ET is then used with crop-specific coefficients to estimate crop water use, and schedule irrigation based on crop need. Because of the critical reliance of sound ET estimations on weather data, we have established quality assurance protocols to test the accuracy, consistency, and reliability of the measured weather data. This research describes procedures developed to test and enhance the integrity of the weather data. Relative humidity is particularly susceptible to measurement error, though alternative methods of estimating vapor pressure deficit can reliably substitute. While most climatological parameters can be estimated by neighboring stations, the discrete nature of rainfall both temporally and spatially make such estimation infeasible. Alternative methods of establishing weather data of high integrity for ET calculations are given.

Technical Abstract: Increasingly, producers are relying on irrigation to enhance yields and improve return on investment. The greater demand for ground water to support supplemental irrigation in the Mississippi River Alluvial Flood Plain has resulted in a decline in the aquifer, and a subsequent implementation of more stringent regulations for well permits. Given the concerns of ground water depletion, producers can clearly benefit from a tool that would indicate when they need to irrigate, and how much. However, no easy to use irrigation scheduling tools have been developed and calibrated for the humid, high-rainfall environment of Mississippi. Researchers at USDA-ARS and Mississippi State University are developing a daily irrigation scheduling tool that calculates reference evapotranspiration (ET) from weather data to establish a crop water balance which will indicate the need to irrigate, or not. The weather parameters required for the calculation of ET include temperature, vapor pressure (determined from relative humidity and temperature), wind, and solar radiation. Rainfall and irrigation are also needed to track water balance in the field. Accurate determination of ET requires weather data that are consistent, accurate, and reliable. Errors in archived weather records are introduced from several sources, including sensor limitations and failures, instrument siting and deployment issues, lack of timely cleaning or re-calibration of instruments, data logger failures or programming errors, data transmission problems, and human errors. Further errors in the calculation of ET can result from mixing units in the calculations (e.g, degrees Fahrenheit and Celsius), incorrectly converting units, or failing to adjust wind measurements to a common height. Since the archived data has not undergone any systematic assessment of data quality, potential users must conduct their own rigorous quality assurance before using the data to calculate ET. For the purposes of this decision support tool, algorithms will need to be written and applied to daily data to flag and remove erroneous or questionable data, properly convert units, and adjust sensors to a common reference height before daily calculations of ET can be performed. The research described here identifies common sources of errors in the available Mississippi weather station data, and develops statistical and range limit tests to identify erroneous records. Such quality assessment and control procedures are essential to ensure that the values produced by the daily irrigation scheduling tool are reasonable and useful for decision support. The protocols described in this research will establish accurate data needed for calculation of reference crop ET for irrigation scheduling and crop management. Developing technologies to manage the agricultural water resources of the humid Mid-South United States will provide means for agricultural producers to increase water use efficiency and mitigate ground water depletion.