H11F-0841
Modeling Spatial Recharge in the Arid Southern Okanagan Basin and Impacts of Future Predicted Climate Change
Groundwater systems in arid regions will be particularly sensitive to climate change owing to the strong dependence of evapotranspiration rates on temperature, and potential shifts in the precipitation amounts and timing. In this study, future predicted climate change from three GCMs (CGCM1 GHG+A, CGCM3.1 A2, and HadCM3 A2) are used to evaluate the sensitivity of recharge in the Oliver region of the Okanagan Valley, south- central British Columbia, where annual precipitation is approximately 300~mm. Temperature data were downscaled using Statistical Downscaling Model (SDSM), while precipitation and solar radiation changes were estimated directly from the GCM data. Results for the region suggest that temperature will increase up to 4°C by the end of the century. Precipitation is expected to decrease in the spring, and increase in the fall. Solar radiation may decrease in the late summer. Shifts in climate, from present to future-predicted, were applied to the LARS-WG stochastic weather generator to generate daily stochastic weather series. Recharge was modeled spatially using output from the HELP hydrologic model applied to one-dimensional soil columns. An extensive valley-bottom soil database was used to determine both the spatial variation and vertical assemblage of soil horizons in the Oliver region. Soil hydraulic parameters were estimated from soil descriptions using pedotransfer functions through the ROSETTA program. Leaf area index (LAI) was estimated from ground-truthed Landsat 5 TM imagery, and surface slope was estimated from a digital elevation model. Irrigation application rates were modified for each climate scenario based on estimates of seasonal crop water demand. Daily irrigation was added to precipitation in irrigation districts using proportions of crop types along with daily climate and evapotranspiration data from LARS-WG. The two dominant crop classes are orchard (including peaches, cherries and apples) and vineyards (grapes). Recharge in irrigated areas is significantly higher, with irrigation return flow between 25--58%. Recharge results show a general increase of annual recharge, with the peak recharge shifting from March to February. Lower recharge rates and higher potential evapotranspiration rates are expected in the summer. The minor increase of annual recharge in future predicted climate states is due the shift of peak recharge from increased temperature. Growing season lengths, as determined from growing degree day accumulation, are expected to lengthen by 3--4 weeks by the 2080s.
H11F-0842
Million-Year Estimates of Net Infiltration at Yucca Mountain, Nevada
The performance period for the potential high-level radioactive waste repository at Yucca Mountain, Nevada, may extend to one million years. Assessments of repository performance may use a stylized steady-state representation for deep percolation fluxes after 10,000 years, while considering the uncertainty in the steady-state value. A procedure for estimating million-year-average deep percolation integrates time sequences of areal- average net infiltration estimates over potential future time-varying climate sequences. Two estimates of million- year-average deep percolation were developed from two independent estimates of future climate sequences, both based on correlating climate (in the form of mean annual precipitation and temperature) during past glacial cycles to the orbital characteristics of the Earth and projecting the orbital characteristics into the future. Correlations based on core data, glacier extent, lake stands, treeline variation, and vegetation species composition provide bases for relating climate to the extent of continental glaciation. Despite the independent assumptions and data sources, both approaches to estimating climate yield similar estimates of million-year- average future precipitation and temperature. A numerical model for net infiltration, which compares well with regional and site estimates for net infiltration, provides the link between climate and areal-average deep percolation. Analyses using the net infiltration model suggest that million-year-average net infiltration is expected to be approximately 3 times greater than at present using both sets of climate estimates. The analysis found that the mean and variance of estimated future million-year-average areal-average net infiltration is reduced by less than 10 percent and less than 20 percent, respectively, from the case using uncertain time-varying climate to the case with steady and certain climate, regardless of the climate sequence used. The results imply that most of the uncertainty in estimating future net infiltration can be attributed to the uncertainty in estimating net infiltration for individual climate states. Small systematic increases in expected long-term-average net infiltration arise from uncertainty in the climate sequences that may occur over glacial cycles and from climatic variability over glacial cycles. This paper is an independent product of the CNWRA and does not necessarily reflect the view or regulatory position of the NRC.
H11F-0843
Climate Change Effects on Groundwater Recharge East of Yucca Mountain
In order to better understand the general flow system and climate-induced changes in recharge around Fortymile Wash, near Yucca Mountain, groundwater geochemical data from the Amargosa Desert region were analyzed. Also, chloride mass balance (CMB) was applied to drill cuttings from borehole NC-EWDP-22S near Fortymile Wash. Stable isotopic hydrogen-2 and oxygen-18 data indicate that less depleted groundwater is found under the flowpath of Fortymile Wash compared with groundwater perpendicular to the wash, and in the lower end of the wash compared with the source in the canyon. In addition, water isotope data under Fortymile Wash plot below the global meteoric water line (GMWL), suggesting low evaporation before infiltration. Total dissolved solids (TDS) and chloride (Cl) concentrations are lowest in the groundwater along the wash indicating less rock/water interaction and low evaporation prior to infiltration. In consequence, stable isotope, TDS and Cl data are most consistent with a pattern of infiltration and recharge of surface runoff subsequent to runoff-generating storms. Carbon-14 data corrected with carbon-13 data presents ages between 8,000 years before present (BP) in the upper canyon region and 14,000 years BP in the lower region near the Amargosa Desert. This range in ages corresponds to the end of the Pleistocene and early Holocene epochs. In contrast, groundwater adjacent to Fortymile Wash appears to be older than that beneath the wash. Furthermore, the trend of groundwater age increase and further stable isotope depletion beneath Fortymile Wash with increasing distance from the canyon suggests that the average reach of recharge and runoff events diminished over time as the climate became warmer and dryer. CMB results present two different pore velocities, the slower one nearer to the surface and with the transition occurring between 6 and 26 meters in depth corresponding to 8,5000 and 11,000 BP. Considered together, these facts suggest that ground water under Fortymile Wash is not derived primarily from migration of adjacent ground water, as indicated by coarse contoured water levels, but instead from past focused infiltration that diminished due to a changing climate.
H11F-0844
The Impact of the Climate Change on Taipei Groundwater Resources
Climate change has become a serious issue around the world. The variate of precipitation and temperature will influence both streamflows and the recharge of groundwater, which may further influence the availability of water resources. In this paper, we propose a methodology to evaluate the impact of the climate change on groundwater resources. The methodology includes two simulation models, the GWLF model and MODFLOW, and two heuristic algorithms, respectively. The GWLF model and MODFLOW are used to simulate streamflows and groundwater, respectively. The simulated annealing algorithm is applied to identify the historical recharge rate and then the genetic programming is used to construct functions describing relationships between streamflows and the identified recharge rate. To evaluate the climate change impacts on groundwater, streamflows are simulated first based on different climate scenarios which are derived from GCMs. The simulated streamflows are then applied to estimate the recharge rates and furthermore the impacts on groundwater can be evaluated. The Taipei basin, which is an important groundwater aquifer in North Taiwan, is chosen as a study site to verify the proposed methodology. Our results evaluate the impact of climate change on groundwater resources and can provide the useful information for the future conjuncted use of surface and groundwater resources to strengthen adaptive capacity for water resources management.
H11F-0845
Application of GIS Based Tools for Groundwater Recharge and Evapotranspiration Estimation: Arc-Recharge and RIPGIS-NET
Water managers are increasingly concerned about the potential impact of climate variability and change on groundwater resources. Climate impacts on groundwater resources are primarily determined by altering the amount of recharge and evapotranspiration (ET). Typically, groundwater models employ temporally static recharge or ET rates with limited spatial variability across the basin. As a result most groundwater models cannot be used to assess the impacts of climate on groundwater resources. A primary challenge addressing this shortcoming is the need for spatially and temporally explicit recharge and ET model inputs. Geographic Information Systems (GIS) and spatially explicit data can be applied to develop these improved model inputs by quantifying and distributing recharge and ET across the model domain. Two ArcGIS desktop applications were developed for ArcGIS 9.2 to enhance recharge and ET estimation- Arc- Recharge and RIPGIS-NET. Arc-Recharge an ArcGIS 9.2 custom application is developed to quantify and distribute recharge along MODFLOW cells. Using spatially explicit precipitation data and Digital Elevation Model (DEM), Arc-Recharge routes water through the landscape and distributes the recharge to the appropriate groundwater model cells. RIPGIS-NET is an ArcGIS custom application that was developed to provide parameters for the RIP-ET package. RIP-ET is an improved MODFLOW ET module that simulates ET using a set of eco-physiologically based ET curves. RIPGIS-NET improves alluvial recharge estimation by providing spatially explicit information about the riparian/wetland ET. Application of Arc-Recharge and RIPGIS-NET in groundwater modeling enhances recharge and ET estimation by incorporating temporally and spatially explicit data. Using such tools, assessment of climate variability on groundwater resources will be enhanced.
H11F-0846
Palaeo-recharge in Arid Northwestern China: Geochemical Inferences and Application to Predicting Hydrologic Response to Climate Change
Recharge investigations based on ionic and isotopic properties of groundwater and vadose zone moisture have revealed temporal heterogeneity in recharge to the hyper-arid Badain Jaran Desert in northwestern China, which is situated near the contemporary northernmost extent of the East Asian Summer Monsoon (EASM). Chloride- based records indicate that rates of potential recharge have varied by at least a factor of three over the last 700 years in approximately century-scale cycles. Correspondence with palaeoclimatic proxy evidence from more humid regions of western and central China suggests that variations in the strength of the EASM are responsible for the observed changes. On longer timescales, hydrologic impacts of late-Holocene aridification on the desert's shallow aquifer system and related oasis lakes are apparent from total dissolved solids and δ18O of radiocarbon dated groundwaters, as well as diminishing piezometric levels. These results may aid in prediction of desert hydrologic response to future climatic changes because 1) a link between monsoonal rainfall and groundwater recharge rate is demonstrated and 2) general circulation models predict marked changes in Asian monsoon behavior over the 21st Century.
H11F-0847
Simulation of Paleoclimate Impacts on Groundwater Elevations in The Southern High Plains Of Texas