It is essential to standardize the definitions and approaches to quantifying various irrigation per formance measures. The ASCE Task Committee on Defining Irrigation Efficiency and Uniformity provides a comprehensive examination of various performance indices such as irrigation efficiency, application efficiency, irrigation sagacity, distribution uniformity, and others. Consistency is provided among different irrigation meth ods and different scales. Clarification of common points of confusion is provided, and methods are proposed whereby the accuracy of numerical values of the performance indicators can be assessed. This issue has two companion papers that provide more detailed information on statistical distribution uniformity and the accuracy of irrigation efficiency estimates.

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Nonuniform field runoff can reduce the effectiveness of filter strips that are a uniform size along a field margin. Effectiveness can be improved by placing more filter strip where the runoff load is greater and less where the load is smaller. A modeling analysis was conducted of the relationship between pollutant trapping efficiency and the ratio of filter strip area to upslope contributing area, i.e., buffer area ratio. The results were used to produce an aid for designing filter strips having consistent effectiveness along field margins where runoff load is nonuniform. Simulations using the process-based Vegetative Filter Strip Model show that sediment and water trapping efficiencies of a filter strip increase nonlinearly as the buffer area ratio gets larger. Site characteristics, including slope, soil texture, and upslope soil cover management practices, help to define this relationship more accurately. Using the Vegetative Filter Strip Model simulation results, a graphical design aid was developed for estimating the buffer area ratio required to achieve specific trapping efficiencies for different pollutants under a broad range of agricultural site conditions. A single graph was produced showing simulation results for seven scenarios as a family of lines that divide the full range of possible relationships between trapping efficiency and buffer area ratio and into fairly even increments. Simple rules guide the selection of one line that best describes a given field situation by considering slope, soil texture, and field cover management practices. Relationships for sediment-bound and dissolved pollutants are interpreted from the Vegetative Filter Strip Model results for sediment and water. The design aid is easy to use, accounts for several major variables that determine filter strip performance, and is based on a validated, process-based, mathematical model. The use of this design aid will enable a more precise fit between filter size and runoff load where runoff from agricultural fields is nonuniform.Key words: models-nonpoint source pollution-precision conservation-surface runoffvariable-width buffers-vegetative buffers-water quality-watershed planning Filter strips are commonly installed for improving and protecting water quality in agricultural watersheds. Filter strips (Code 393) reduce the load of sediment, nutrients, and other pollutants in overland runoff from fields by promoting infiltration and sediment deposition (Haan et al. 1994;USDA 1997). Typically, they are designed to have a constant width (parallel to flow) along a field margin, and maximum pollutant trapping efficiency is achieved when field runoff is uniformly dispersed across the entire filter strip (USDA 1997). Several design methods have been developed for determining the appropriate width for a filter strip treating spatially uniform runoff (see review in Dosskey et al. 2008). However, researchers have observed that surface runoff commonly concentrates in fields and flows mainly through only small portions of filter strips...

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Climate and vegetation strongly influence the water cycle on local to regional scales. A change in the surface energy and water balance, especially in dry climatic regions, can have a significant impact on local water availability and, therefore, water resource management. The purpose of this study is to quantify the energy and water balance of a riparian wetland in a subhumid region of the central US, as well as the role of seasonal climate variability and vegetation phenology. The site is located in the Republican River basin in south-central Nebraska, where decreases in streamflow have been observed in recent decades. In an effort to reduce consumptive water use from evapotranspiration (ET), and thereby reclaim surface water, invasive species such as Phragmites australis have been removed throughout the riparian corridor of the river basin. In this study, we used energy/water balance monitoring stations, a Large Aperture Scintillometer (LAS), and numerous water and soil temperature probes to determine the energy and water balance during the 2009 growing season (April 11−October 3). Sensible heat flux was measured using the LAS, while ET was calculated as a residual of the energy balance (i.e., net radiation minus sensible heat flux and heat storage rates in the canopy, water, and soil). Rigorous quality control and uncertainty analyses were performed, and comparisons were also made with ET rates calculated via the simpler Priestley-Taylor method. Results of the energy budget analysis indicate that the average ET rate for the wetland during the growing season was 4.4 mm day −1 , with a maximum daily rate of 8.2 mm day −1 (occurring on June 29). Precipitation during the same 176-day period averaged 2.7 mm day −1. Net radiation and vegetation phenology were found to be the two largest drivers of seasonal variability in ET. Sensible heat flux was significantly larger than latent heat flux early in the season, when standing vegetation in the wetland was still dry and brown. By late May and early June, however, Bowen ratios had declined well below 0.5 in response to greener and more abundant vegetation, higher transpiration rates, and reduced sensible heat flux. Heat storage rates in the wetland were dominated by changes in water temperature (as compared to soil or canopy heat storage) and comprised a significant portion of the hourly energy balance. On daily mean timescales, changes in the rate of heat storage corresponded to ~13% of the variability in net radiation, while for the season-long average, the heat storage term was found to be essentially negligible. The Priestley-Taylor equation provided a reasonable estimate of ET during the height of the growing season but significantly overestimated ET during the beginning of the season (since it could not account for large sensible heat fluxes from the dry vegetation). Analysis of the wetland water balance showed seasonal variations in water level that were similar to changes in cumulative water inputs (i.e., precipitation minus ET). Portions of the season when the...