H14E-01 INVITED
Options for Monitoring Climate-driven Recharge Changes in Western Mountains
Ground-water supplies have long been used as fallback supplies during droughts and other lean times, and thus it has been natural to view them as fall backs if deficits develop as a result of long-term climate change. However, ground-water supplies may not be immune to detrimental influences from climate changes of the sort projected under the influence of increasing atmospheric greenhouse-gas concentrations. An example of a possible detrimental influence is the potential for significant reductions in mountain-based recharge in the western USA as mountain snowpacks decline in response to warming trends. Most recharge to western aquifers comes from snowmelt and, as snowpacks diminish, opportunities for recharge are at risk of serious declines. A multidisciplinary workshop, funded by the USGS and California Energy Commission, was held in July 2007 to discuss options for long-term monitoring of mountain recharge variations and changes in California and Nevada. The meeting polled hydrologists, geochemists, geophysicists, and biologists about methods for characterizing western recharge fluctuations. In particular, the meeting asked whether current methods exist for characterizing changing recharge rates and mechanisms that could be used in a geographically distributed, long-term monitoring network. Hydrologic methods, including monitoring wells and streambed methods, have been underutilized in mountain-recharge settings and offer important, but at-present uncommon, views of the influences and timing of recharge in western mountains. Among geophysical methods, repeated microgravity measurements appear to hold the most promise for tracking recharge fluctuations. The method has been used to measure recharge episodes on alluvial fans and basin floors, but has not been applied in mountain settings, so protocols and experience are needed prior to widespread mountain deployment. Biological observations at springs and seeps have the potential to provide important evidence of changing hydrologic conditions associated with recharge fluctuations but, to date, have been used thusly mostly at low-altitude springs. Geochemical methods—including major ions, isotopes and dissolved gases—have been used to characterize recharge mechanisms, rates, and pathways in western mountains. Studies to date have focused on characterizing spatial patterns and differences, but repeated sampling over the course of years has the potential to also characterize temporal variations. Although the general sense was that more research would be necessary to design an operational monitoring network using combinations of these methods, several opportunities were identified. In the Sierra Nevada, a number of extensively instrumented research watersheds were identified that do not yet include observations of ground water and recharge. These watersheds could be augmented with wells, streambed sensors, repeat geophysical surveys, and geochemical censuses to help close their water budgets and as opportunities for learning how to use these methods for recharge-variation monitoring. Elsewhere, historical surveys of hydrology, biology, and geochemistry at mountain springs and selected wells/mines in Nevada and California could be repeated regularly as a foundation for detection of recharge changes and as a complement to the kinds of focused studies that may grow from the research watersheds and basins.
H14E-02 INVITED
Unsteady Climate, Groundwater Recharge, and Human Influence
Recharge is arguably the starting point of the groundwater hydrologic cycle. It marries above-ground hydrologic and climatic processes –runoff, precipitation, evapotranspiration- with groundwater flow and biochemical dynamics. This paper focuses on unsteady climate and ground water recharge linkages. Unsteady climate by virtue of its seasonal and inter-annual fluctuations or by shifts in the earth-atmosphere's radiative budget caused by secular forcing. Recharge is primarily caused by spatially diffuse percolation or by streamflow seepage. These two mechanisms are briefly reviewed. Examples of regional recharge governed by unsteady climate and affected by unsteady population are presented. Questions are raised about climate-recharge-human feebacks, and adaptation possibilities are proposed.
H14E-03 INVITED
Sources of uncertainty in climate change impacts on groundwater recharge
This paper assesses the significance of the many sources of uncertainty in future groundwater recharge estimation, based on lessons learnt from an integrated approach to assessing the regional impacts of climate and socio-economic change on groundwater recharge in East Anglia, UK. Many factors affect simulations of future groundwater recharge including changed precipitation and temperature regimes, coastal flooding, urbanization, woodland establishment, and changes in cropping, rotations and management practices. Stochastic modelling of potential recharge showed median annual recharge decreasing under a High emissions future from 75 mm (1961-90) to 56 mm in the 2020s and 45 mm in the 2050s. However, the median values for individual simulations ranged from 46-75 mm (2020s) and 30-71 mm (2050s) highlighting a decreasing but uncertain trend. The impacts of (and uncertainty in) the climate scenarios are generally regionally more important than those of the socio-economic scenarios. However, locally, the impacts of the socio-economic scenarios can be significant, especially where there are large increases in urbanization, agricultural land cover, bioenergy production, or agricultural management practices. For example, management of soil conditions can increase potential groundwater recharge by around 5 %, but poor management can further reduce potential recharge by up to 15 %. The paper will demonstrate that to focus on the direct impacts of climate change is to neglect the potentially important role of policy, societal values and economic processes in shaping the landscape above aquifers. If the likely consequences of future changes of groundwater recharge, resulting from both climate and socio-economic change, are to be assessed, hydrogeologists must increasingly work with researchers from other disciplines, such as socio-economists, agricultural modellers and soil scientists
H14E-04
Climate Influences on Groundwater Recharge: Implications for Western Groundwater and Surface Water Resources in the Face of Climate Change
Groundwater is a vital resource in the western USA, accounting for over a quarter of total supplies and irrigation uses. In addition, groundwater is a major contributor to surface-water resources, sustaining baseflows throughout the year, and contributes important fractions of streamflows even during high-flow periods following rainfall and snowmelt. Because mountains are generally cooler and wetter than adjacent basins, groundwater in the West is derived mostly from mountain precipitation. Large infiltrations of water are required to break through the region's thick unsaturated zones. Because snowpacks store and then release precipitation from several storms at once, snowmelt contributes disproportionately more recharge than does rain. Warming temperatures have already caused declines in Western snowpacks and earlier flows in melt-fed streams. Current projections of future climate suggest that these trends will continue. Snowline elevations are expected to rise, reducing snow-covered areas in western mountains, and decreasing the amount of snow in areas where snowpacks remain. If so, mountain-block recharge also may also decline, as recharge areas shrink and snow available for melt generation dwindles. Declines in mountain recharge triggered by loss of snowpack would have immediate impacts on mountain water resources, including low flows and stream temperatures, and may also have serious impacts on long-term ground-water supplies in surrounding basins. Although recharge that supplies mountain groundwater may decline, much of this unrecharged water may run off onto fans and basins, increasing recharge beyond the mountains. However, if the water that is not recharged in the mountains is mostly evapotranspired from the mountain soils, the overall recharge (mountain plus basin) may decline. Changes in temperature will bring concomitant changes in water temperatures, and thus in streambed conductance and leakance; changes in dominant vegetation may also occur. All these factors can play a role in affecting groundwater recharge, and none are well understood or predicted at present. Declines in recharge triggered by warming could seriously impact ground-water supplies and surface-water resources to which ground water contributes. Given ground water's crucial role in western water, potential impacts of warming on recharge deserve more attention than they have received to date, preferably from a long-term monitoring system incorporating multidisciplinary observations.
H14E-05
Groundwater Recharge Evaluation in Semi-Arid Northeast Mexico in Response to Projected Climate Change
This research evaluates the effects of projected climate change on mountain recharge in the semi-arid Cuatrocinegas Basin (CCB) of northeast Mexico. The CCB UNESCO Biosphere Reserve is located in Coahuila, Mexico (~27° N, ~102° W) and includes > 500 springs that discharge from a regional flow system to wetlands with > 70 endemic species and to an irrigation network. This study tests the hypothesis that projected climate changes will reduce CCB recharge. In CCB, ~75% of annual precipitation (~220 mm at 700 m, ~400 mm at 2350 m) falls between May and October and ~40% falls during the North American Monsoon in June, July, and August. Environmental isotopes indicate aquifer residence times of > 50 years. Stable isotopes (O and H) show that mountain precipitation (at an elevation of ~1170 to 2350 m) dominates groundwater recharge. Recharge is insignificant at lower- elevation valleys that cover the majority of the study area due to high evapotranspiration rates. A Cl--balance water-budget recharge analysis estimates a spatially distributed recharge rate of ~1 to 3% of precipitation to provide at least 35x106 m3/year spring discharge (as measured in canals that drain dozens of springs). IPCC AR4 climate projections predict an annual temperature increase of 3.0 to 3.5°C and an annual precipitation decrease of 5 to 10% for Subregion CNA (located adjacent to CCB) by 2099. During June to August, models project a temperature increase of 3.5 to 4.0°C and a precipitation increase of 0 to 5%. Although global and regional circulation models evaluate mountain regions poorly, a first-order evaluation of climate projections on CCB recharge is needed input to develop effective long-term groundwater management policies. Climate projections suggest that the minimum elevation at which recharge occurs in CCB may increase by ~615 m to 1785 m, which would limit recharge to the highest mountain elevations. If annual precipitation is reduced by 5 to 10% and temperatures increase as predicted, recharge may be significantly reduced, but relatively long aquifer flow paths may delay these effects on spring flow. Ultimately, a combination of reduced mountain recharge and higher valley evapotranspiration would reduce the size of groundwater-dependent wetlands and limit agricultural diversions.
H14E-06
Palaeo-Recharge Impact on Aquifer Hydrodynamics: Paris Basin Case
Here we investigate the response of the Paris basin groundwater system to variations in its hydrodynamic boundary conditions induced by past climate and geodynamic changes over the last five million years. For the purpose of this work, a three-dimensional transient modeling of the Paris basin aquifer/aquitard system was developed using the code NEWSAM (Paris School of Mine). The geometry and hydrodynamic parameters of the model originate from a basin model, NEWBAS (Paris School of Mine), built to simulate the geological history of the basin. Geomorphologic evolution is deduced from digital elevation model analysis, which allows estimating of river-valley incision and alpine uptlif. Climate forcing results from palaeoclimate modeling experiments using the LMDz atmospheric general circulation model (Pierre Simon Laplace Institute) with a refined spatial resolution, for the present, the Last Glacial Maximum (21 kyr BP) and the Middle Pliocene (~3 Myr). The water balance is computed by the distributed hydrological model MODSUR (Paris School of Mine) and provides recharge value to aquifer at each time step. We present the simulated evolution of piezometric heads in the system in response to the altered boundary conditions. For the present, the comparison of head patterns between steady state and time dependent simulation shows little differences for aquifer layers close to the surface but suggests a transient state of the current system in the main aquitards of the basin and in the deep aquifers, characterized by abnormally low fluid potentials. The dependence of the boundary-induced transient effects on the hydraulic diffusivity is illustrated by means of a sensitivity study. Time dependant change of recharge induces rapid inversion of leakage orientation through the multilayered aquifers that may induce modification of groundwater quality.
H14E-07
Evaluating the effects of a +3C climate change with a regional recharge model
A +3C change in air temperature, a conservative average increase suggested by ongoing climate change studies, was evaluated using a monthly regional-scale water-balance model to estimate changes in snow accumulation and melt, recharge, runoff, and evapotranspiration for 5 southwestern U.S. states. Model calibrations to current conditions include potential evapotranspiration, radiation, and snow cover data, and recharge estimates using various methods. Steady-state and transient 30-yr conditions were modeled to investigate changes in timing, volume, and the spatial distribution of the parameters at a 270-m spatial resolution using a 30-yr precipitation record. Overall, average annual snow accumulation was decreased by approximately 46 percent throughout the study area, with maximum reductions in California and minimum reductions in the southern desert regions. Average reductions over the entire study area of 4 percent of in-place recharge and 8 percent of runoff were accompanied by a 20-percent increase in evapotranspiration, illustrating how the timing of snow presence and melt results in increased opportunity for evapotranspiration processes to occur. Even with a simplistic, across- the-board increase in air temperature throughout the west, the magnitude of the effects in areas that develop winter snowpacks are most notable in the Sierra Nevada and the Trinity Mountains of the Klamath River Basin, and less so in the Colorado Rockies, a result that emphasizes the potentially significant consequences of climate change on water resources in California. Regional-scale approaches to modeling the influences of climate change can be readily and expeditiously used to investigate the relative changes in hydrologic processes among large regions for prioritizing management of water resources in the western U.S.
H14E-08
Recharge response to natural climate variability on interannual to multidecadal timescales
Climate change, whether caused by natural variability or human activity, has important implications for recharge and the sustainability of ground-water resources. Reliable predictions of ground-water sustainability under future climate change will require a better understanding of the role that natural variability caused by climate cycles on interannual to multidecadal timescales has in controlling spatiotemporal changes in recharge. Climate cycles on these timescales has been shown to partially control patterns in precipitation and air temperature, as well as streamflow and drought, which in turn can affect evapotranspiration and ultimately recharge rates and mechanisms. Because these climate-varying conditions can augment or diminish human stresses on ground water, the responses in water levels and ground-water storage can be dramatic when different climate cycles lie coincident in a positive (wet/cool) or negative (dry/warm) phase of variability. Thus, understanding climate cycles on these timescales has particular relevance for management decisions during drought and for ground-water resources close to the limits of sustainability. The objective of this study was to quantify how recharge to the High Plains aquifer (USA) responds to natural climate cycles on interannual to multidecadal timescales. Using singular spectrum analysis of long-term hydrologic time series, the signal of ground-water pumping was removed and natural variations were identified in all tree ring, precipitation, and ground water time series as partially coincident with known climate cycles. These cycles included the El Nino/Southern Oscillation (2 to 6 years), the Pacific Decadal Oscillation (10 to 25 years), and the Atlantic Multidecadal Oscillation (50 to 80 years). Climate-varying recharge and water-level fluctuations were most significantly correlated to the Pacific Decadal Oscillation. Using a novel recharge estimation method, climate varying recharge rates (196 to 476 mm yr-1) were found to be substantially larger than previous estimates of diffuse recharge (0.2 to 110 mm yr-1), indicating the importance of preferential flow during recharge to the High Plains aquifer. The results indicate the importance of interdecadal-climate cycles as controls on rates and mechanisms of climate-varying recharge and support the conclusion that understanding natural climate variability is a necessary step toward predicting ground-water response under climate change. Such understanding may help managers to better plan for the long-term sustainability of ground-water resources.