Threat Assessments on California Rangelands

Ratio of Recharge to Runoff for each 30-year climate period

Example recharge runoff Map

These maps display the ratio of average recharge to average runoff for each watershed for the present-day climate period and for three future climate periods. The present-day climate period is 1981-2010 and the future climate periods include 2010-2039, 2040-2069 and 2070-2099. Recharge:runoff is provided for two climate projections for each of the three IPCC-SRES scenarios – A1B, A2 and B1. Watershed boundaries are from the 8-digit Watershed Boundary Dataset ( The ratio of in-place recharge to runoff indicates the mechanisms, including soils, geology, and precipitation patterns that likely control ground-water recharge for a given watershed. A ratio of 0.5 or less indicates that more than twice as much water has the potential to become runoff than to become in-place recharge. A ratio of 2.0 or greater indicates that water has at least twice as much potential to become in-place recharge than to become runoff (Flint and Flint, 2007).

Recharge and runoff were modeled using the U.S. Geological Survey's Basin Characterization Model (BCM), a regional water balance model (Flint et al. 2013, Flint and Flint, 2012). The BCM was run with two statistically downscaled global climate models (GCMs) (a warm, wet future and a hot, dry future) for each emissions scenario. GCMs included variables for minimum or maximum temperatures, which were considered important determinants of vegetation distribution. Table 1 summarizes the GCMs for each emission scenario.

Table 1. Summary of Global Climate Models (GCMs)

Climate models table

These representative projections were downscaled to 270 meter spatial resolution for monthly estimates of precipitation and maximum and minimum air temperature. The BCM used the downscaled precipitation and temperature as well as elevation, geology, and soils to produce 270 meter-resolution maps of potential evapotranspiration, runoff, recharge, CWD, actual evapotranspiration, sublimation, soil water storage, snowfall, snowpack, snowmelt, and excess water.

Thirty-year water year summaries of recharge and runoff were used for this rangelands project. The maps display the ratio of the 30-year averages of recharge to runoff, averaged by watershed area. The recharge:runoff ratio is calculated as: Rch/Run, where Rch = the 30-year recharge average, averaged by watershed area, and Run = the 30-year runoff average, averaged by watershed area.


Flint, L.E., A.L. Flint, J.H. Thorne, and R. Boynton. 2013. Fine-scale hydrologic modeling for regional landscape applications: the California Basin Characterization Model development and performance. Ecological Processes 2:25. Available online at:

Flint, L.E. and A.L. Flint. 2012. Downscaling future climate scenarios to fine scales for hydrologic and ecologic modeling and analysis. Ecological Processes 1:2. Available online at:

Flint, L.E. and A.L. Flint. 2007. Regional analysis of ground-water recharge. In: Stonestrom, D.A., J. Constantz, T.P.A. Ferré, and S.A. Leake (eds). Ground-water recharge in the arid and semiarid southwestern United States. Professional Paper 1703. Reston (VA): U.S. Geological Survey. Available online at: