Change in snowpack (snow water equivalent) in California between 1951–1980 and 1981–2010. The losses mirror worldwide trends everywhere but the highest elevations. USGS.
The problem
Rising air temperatures are changing water supplies worldwide: precipitation timing is shifting, snowpack is shrinking and snowmelt is coming earlier. Landscapes, plants, animals and farms feel it through longer dry seasons (which also raise demand for water), more extreme storms, fewer chilling hours for fruit trees, and higher snowlines.
Managers planning for the future have mostly relied on global climate model projections of temperature and precipitation — too coarse for regional and local decisions. The USGS has developed tools that combine climate data with detailed local environmental data to show how watersheds and landscapes will respond.
What managers need:
- Recharge and runoff — how much water, when, and how extreme.
- Climatic water deficit — how much water plants would use if it were available: a measure of irrigation demand and landscape stress.
- Where these processes happen across a watershed, for planning and infrastructure.
Runoff in California, 1981–2010, estimated with the Basin Characterization Model. USGS.
The Basin Characterization Model
The BCM calculates the water entering and leaving a landscape, usually monthly across large regions (daily models exist for small watersheds). The land is divided into grid cells, and each cell's water balance is solved from its climate inputs:
- Potential evapotranspiration, from solar radiation with topographic shading and cloud cover.
- Snow, as it accumulates and melts.
- Water moving through the soil, which gives actual evapotranspiration and the climatic water deficit (potential minus actual evapotranspiration).
- Depending on the soil and how permeable the bedrock is, surplus water becomes recharge or runoff.
Results can be added up for any watershed or region — baseflow, streamflow and recharge to groundwater — or mapped across the landscape.
How the model moves water through the soil and into bedrock to become recharge or runoff. USGS.
Downscaling
Global climate projections for the 21st century are downscaled to 270-meter cells. At that scale the model captures what really governs the water balance — energy loads and topographic shading — and can use maps of soils, vegetation and land use.
Downscaling a global climate projection from 12 kilometers to 270 meters. USGS.
Calibration: why trust it?
The model is calibrated — tuned to match observations — at basins across the western United States. It first reproduces unimpaired flow in unregulated upstream tributaries, such as Upper Dry Creek in the Russian River basin; then diversions and agricultural demand are added to match impaired flow downstream, as at the Guerneville gage. A final daily calibration for the Russian River at Hopland gives confidence in projecting streamflow into the future.
Calibration in the Russian River basin: (A) flows considered, (B) observed and modeled flow at Hopland. USGS.
How climate change affects water
Timing and amount. Air temperature controls when snow melts, and melt has been earlier in recent years. Whatever the total precipitation, less will fall as snow, and snowpack will not carry water supply as far into the dry season.
Projected April 1 snowpack, Indian Creek basin, through the 21st century. USGS.
Projected change in recharge across the Klamath River basin. USGS.
Extremes. Atmospheric rivers bringing big winter storms, and long droughts, lead to flooding, poorer water quality, harm to agriculture and threats to fisheries.
Photograph by L.E. Flint, USGS.
Infrastructure
Reservoirs, canals and pumps were built around each watershed's dominant hydrologic process. As climate shifts the timing of rain and snowmelt, together with soil depth, it changes how much water recharges groundwater rather than running off:
- With less snow and deep soils, recharge can rise during a compressed wet season and the longer dry season.
- With shallow soils, higher peak flows can increase runoff.
The Russian, Tuolumne, Merced and American basins all depend on reservoirs to carry water through the dry season. The American is projected to gain recharge relative to runoff; the Russian, Tuolumne and Merced could lose recharge — changes that could affect reservoir operations and justify changes in groundwater-storage management.
Projected change in the ratio of recharge to runoff, 1981–2010 to 2071–2100, under one climate projection. USGS.
Landscapes, vegetation and farms
The climatic water deficit — the yearly evaporative demand that exceeds available water — combines climate, energy, drainage and soil moisture in one number. It closely tracks where vegetation grows, and it indicates irrigation demand. Because soils can only store so much of the wet season's rain, the deficit by summer's end rises under every climate scenario — even wetter ones.
Climatic water deficit in (A) San Francisco Bay area watersheds and (B) Brazil's Atlantic Forest, projected for 2080. USGS.
Where it has been used
- Local: Russian River basin (water management and climate change); San Diego watershed (groundwater); Modoc National Wildlife Refuge (water and landscape stress).
- Regional: the Sierra Nevada (wolverine habitat and snowmelt); the western U.S. (links between forest die-off, wildfire severity and water deficit); the Great Basin and Upper Colorado River basins (stress on threatened species, recharge).
- International: the Tigris–Euphrates basin in Iraq; Brazil's Atlantic Forest; small coffee farms in Central America.
It can help prioritize management, map landscape stress or irrigation demand now and in future climates, and find where species may shift and where climatic and hydrologic refuges may lie.
In short
The BCM uses detailed maps of soils, geology and topography with monthly or daily climate to map recharge, runoff, snowpack, evapotranspiration and water deficit from hillslope to region. Calibrated with historical climate and streamflow, it turns climate projections into local hydrologic effects. It models natural (unimpaired) conditions, but can add demands, diversions and reservoir releases, and can feed groundwater-flow models such as MODFLOW and integrated models such as MF-FMP.
Data: the California BCM is available from the California Climate Commons.
Sources
Based on Lorraine E. Flint, Alan L. Flint and James H. Thorne, "Climate change: evaluating your local and regional water resources," U.S. Geological Survey Fact Sheet 2014–3098; a work of the United States government in the public domain. The maps, diagrams, graphs and the author's photograph are taken from the fact sheet's PDF; a landscape photograph credited to S.B. Weiss is not reproduced.
Licence: CC0 1.0 (public domain) · Adapted from pubs.usgs.gov
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