The worldwide trend in air temperatures in Figure 1,
reported by the Intergovernmental Panel on Climate Change (Climate Change 2007,
AR4 www.ipcc.ch), is a catalyst for studies
of the effects of climate change for water resources. Hydrologists and
meteorologists have assumed that the climate was stable – that observations
like stream flows or mean monthly temperatures in a watershed would, for
‘practical purposes’ like reservoir design, be drawn from the same statistical distribution.
Most instrumental records are less than 100 year in length – the first stream
gage in the
The earth’s climate has been very different in geologic time, and has been significantly different even within the current interglacial warm period (the last 10,000 years). Volcanic eruptions affect worldwide temperatures. The assumption that the climate is stable is an argument about rate of change of climate – that even though climate change is certainly occurring, the rate of climate change is so slow that the stable climate assumption is justified.
Within the last ten years additional lines of inquiry have made the stable climate/slow climate change hypothesis doubtful.
Evidence for and possibilities for more rapid climate change comes from;
Water resource systems (reservoirs, aqueducts, irrigation systems) take decades to plan and build. If climate change affects existing facilities or makes new facilities necessary it is imprudent to ignore possible climate change effects.
The extent of ‘warming’ is not clear – models differ among themselves and projections depend on assumptions about future greenhouse gas concentrations. Projections vary geographically – it’s clear that polar regions experience the largest temperature increases. Effects for other meteorological time series; precipitation, potential evapotranspiration, solar radiation and wind are contradictory.
Analysis of climate change with HFAM modeling takes advantage of the detailed processes in the model. For example, temperature change can be specified by time of day and by seasons. Changes in cloudiness or solar radiation at the land surface, changes in wind or potential evapotranspiration, or changes in rain/snow during precipitation events can all be represented.
The hydrological effects of climate change are watershed specific – they depend on topography, elevations, soils and vegetation – and on the physical facilities in the watershed. Denman glacier shows the sensitivity of both snow and glacial growth or depletion to small increments in temperature and precipitation (Denman Glacier).
The following Figures, 2A, 2B and 2C show;
Figure 2A Current Climate
Figure 2B Warmer storms, + 4 deg. F, no change in dry weather temperatures
Figure 2C Increased Solar and Wind, + 10 percent
In these test runs snow pack accumulation is most sensitive to changes in storm temperatures. Snow pack accumulation is less sensitive to increased solar radiation and wind velocities. Results like these provide clues for interpretation of GCM projections. Different sensitivities may occur at other elevations or exposures. In evaluating climate change the devil is in the details.