|Hipps, Lawrence -|
|Neale, Christopher -|
Submitted to: BARC Poster Day
Publication Type: Abstract Only
Publication Acceptance Date: March 23, 2011
Publication Date: April 27, 2011
Citation: Alfieri, J.G., Kustas, W.P., Prueger, J.H., Evett, S.R., French, A.N., Hipps, L.E., Neale, C.M., Dulaney, W.P., Mckee, L.G. 2011. Hot, dry, & windy: The impacts of strongly advective conditions on measurements of evaporative water loss from irrigated croplands[abstract]. Abs. 1, BARC Poster Day. Technical Abstract: As the demand for water has grown in recent years, balancing the needs of urban, rural, and agricultural communities has become formidable task, particularly in the western United States where more than 70% of the freshwater supply is used for irrigated agriculture. To ensure there is sufficient water to meet the water requirements of all of these communities, there has been a concerted effort to enhance conservation by maximizing the efficiency of irrigation and other agricultural water management practices. A key component of this effort is improving our ability to monitor and predict evaporative water loss (EWL) from croplands. Currently, much of the data used to develop and evaluate evaporation models is collected via two methods: lysimetry (LY) and eddy covariance (EC). Each of these methods is built on different theoretical underpinnings and has different set of advantages and disadvantages. Using data collected over irrigated cotton fields during the 2008 Bushland Evapotranspiration and Agricultural Remote Sensing Experiment (BEAREX08), this study sought to achieve two objectives. The first was to characterize the differences in measurements of EWL collected using the LY and EC techniques. The second goal of the study was to identify the underlying cause of the discrepancies between the LY and EC measurements. One unique aspect of this study is that it used data collected under strongly advective, or more simply hot, dry, and windy, conditions. Through a process commonly referred to as the “oasis effect”, evaporation is increased during advective conditions as warm, dry air passes over the cooler, moister field. A comparison of the data showed there to be significant differences between the LY and EC measurements with the measurements from the lysimeters exceeded those from the EC systems by as much as 0.6 mm hr-1 when measured in terms of equivalent depth. While this difference may not seem significant, the cumulative difference in the measured EWL for the 35-day study period was 90 mm, or equivalently 2,354,000 gallons for the 12 acre field. Several factors were found to contribute to the disparity in the measurements. The chief among these, explaining nearly two-thirds of the discrepancy, were differences in the vegetation density over the lysimeters and the remainder of the field. A second contributing factor was the advective conditions which resulted in greater atmospheric demand near the lysimeters compared to the EC systems which were further downwind. Overall, this study illustrates the uncertainty inherent in field measurements and underscores the need to carefully consider the complex factors affecting evaporation and its measurement when using observational data to develop or evaluate models for predicting EWL and assessing agricultural water use.