The resulting model is computationally efficient enough to be applied at large spatial scales and yet yields spatially explicit results that are useful for conservation planners tasked with targeting sub-field scale management practices. In addition to predicting when and where storm runoff will occur, this model uses open source coding (R-programming language, R Core Team, 2013) and information (e.g., USGS and USDA geographical information) in a manner that is easily applicable

to web-based applications. The modeling approach adopted here is similar to that used by the early forms of TOPMODEL (Beven and Kirkby, 1979), STOPMODEL (Walter et al., 2002), and VSLF (Schneiderman et al., 2007) in which GSK126 ic50 the soil- and ground-water budgets are maintained at the watershed scale (Fig. 1) while storm runoff is distributed according to topographic position within the watershed. The soil water budget that forms the backbone of the model was first proposed by Thornthwaite and Mather (1955). Daily modeled soil water and evapotranspiration (ET) are based on soil water status and potential evapotranspiration (PET): equation(1a) SWd=SWd−1expId−CcPETdAWC for   Id−CcPETd<0 equation(1b) SWd=SWd−1+(Id−CcPETd)−D for   Id−CcPETd≥0SWd=SWd−1+(Id−CcPETd)−D for   Id−CcPETd≥0

equation(1c) D=SWd−1+(Id−CcPETd)−AWC for   SWd−1+(Id−CcPETd)>AWCD=SWd−1+(Id−CcPETd)−AWC for   SWd−1+(Id−CcPETd)>AWCwhere SWd is Selleckchem Anti-diabetic Compound Library soil water depth on day d (mm), AWC is the watershed-wide average available water capacity of the soil (mm), Id is water input on day d (rain + snow melt − Qd) (mm), Cc is a generalized crop coefficient to scale PET under various effective vegetative covers (adopted from Shuttleworth, 1992), D is drainage to the groundwater (mm), and Qd is storm runoff on day d (mm). Storm runoff is estimated using Eq. (2) (discussed in the next paragraph). The watershed-average

HSP90 AWC is calculated from the area-averaged AWC-percentage (mm water per mm of soil depth) and soil depths from the NRCS SSURGO database ( NRCS, 2013). Daily PET is calculated using the Priestley–Taylor (1972) equation using daily maximum and minimum air temperature to estimate net radiation ( Archibald and Walter, 2013). A similar method is used to model daily snow ( Walter et al., 2005 and Fuka et al., 2012). Baseflow is modeled using a linear reservoir model adopting an average regional coefficient of 0.1 day−1 based on recession flow analysis of streams in the northeastern US ( Frankenberger et al., 1999). Storm runoff is estimated using the SCS Curve Number equation (e.g., USDA-NRCS, 2004): equation(2) Qd=Pd2Pd+Sdwhere Qd is runoff on day d (mm), Pd is the effective precipitation and/or snow melt (mm) for that day defined as rain plus snowmelt minus an initial abstraction – here we use initial abstraction = 0.