Hydrologic Budgets across the Long-Term Agroecosystems Research Network.

Quantification of the various components of the hydrologic budget at a site (precipitation, evaporation, runoff,…) gives important indications about major and minor hydrologic processes controlling field and watershed scale response. Hydrologic budgets are needed prior to assessment of potential changes attributed to climate or land management. The objectives of this study were to: 1) develop hydrologic budgets for watersheds (> 400 ha) and/or fields (< 100 ha) at the 18 USDA Long-Term Agroecosystems Research (LTAR) network sites using measured data, model estimates, and/or relevant published literature values, for periods ranging from 2 to 50 years, and 2) quantify the uncertainty of these fluxes. Land cover in the fields consisted of grass and shrubs on rangeland and continuous or rotation grain and seed crops on cropland. Uncertainty of each component was quantified based on measurement or estimation techniques and years of data available. While precipitation and evapo-transpiration (ET) ranged from around 250 mm in the Southwest to > 1000 mm in the Southeast, the ET coefficient (ET/[precipitation+irrigation]) was fairly constant for fields with similar land cover within the same climatic region: 0.73 on crop sites with humid climate, and 1.05 for grassland in semi-arid regions. These ratios were more variable for watersheds than for fields, and often but not always smaller. Field-scale water losses due to surface and subsurface runoff, drainage, and percolation ranged from 0 mm in semi-arid regions to 373 mm in Southeast US and varied with precipitation and soil type. Field sites could be grouped according to the water loss coefficients (water losses/[precipitation + irrigation]): around 0.30 for sites east of the Mississippi (all cropland), 0.15 to 0.25 for cropland sites west of the Mississippi, and close to 0 for western rangeland sites.