10As the world's largest distributed store of freshwater, groundwater plays a central role in 11 sustaining ecosystems and enabling human adaptation to climate variability and change. 12The strategic importance of groundwater to global water and food security will intensify 13 under climate change as more frequent and intense climate extremes (droughts, floods) 14 increase variability in soil moisture and surface water. Here we critically review recent 15 research assessing climate impacts on groundwater through natural and human-induced 16 processes as well as groundwater-driven feedbacks on the climate system.

Six land surface models and five global hydrological models participate in a model intercomparison project [Water Model Intercomparison Project (WaterMIP)], which for the first time compares simulation results of these different classes of models in a consistent way. In this paper, the simulation setup is described and aspects of the multimodel global terrestrial water balance are presented. All models were run at 0.58 spatial resolution for the global land areas for a 15-yr period (1985-99) using a newly developed global meteorological dataset. Simulated global terrestrial evapotranspiration, excluding Greenland and Antarctica, ranges from 415 to 586 mm yr 21 (from 60 000 to 85 000 km 3 yr 21 ), and simulated runoff ranges from 290 to 457 mm yr 21 (from 42 000 to 66 000 km 3 yr 21 ). Both the mean and median runoff fractions for the land surface models are lower than those of the global hydrological models, although the range is wider. Significant simulation differences between land surface and global hydrological models are found to be caused by the snow scheme employed. The physically based energy balance approach used by land surface models generally results in lower snow water equivalent values than the conceptual degreeday approach used by global hydrological models. Some differences in simulated runoff and evapotranspiration are explained by model parameterizations, although the processes included and parameterizations used are not distinct to either land surface models or global hydrological models. The results show that differences between models are a major source of uncertainty. Climate change impact studies thus need to use not only multiple climate models but also some other measure of uncertainty (e.g., multiple impact models).

[1] The growth of vegetation is affected by water availability, while vegetation growth also feeds back to influence regional water balance. A better understanding of the relationship between vegetation state and water balance would help explain the complicated interactions between climate change, vegetation dynamics, and the water cycle. In the present study, the impact of vegetation coverage on regional water balance was analyzed under the framework of the Budyko hypothesis by using data from 99 catchments in the nonhumid regions of China, including the Inland River basin, the Hai River basin, and the Yellow River basin. The distribution of vegetation coverage on the Budyko curve was analyzed, and it was found that a wetter environment (higher P/E 0 ) had a higher vegetation coverage (M) and was associated with a higher evapotranspiration efficiency (E/E 0 ). Moreover, vegetation coverage was related not only to climate conditions (measured by the dryness index DI = E 0 /P) but also to landscape conditions (measured by the parameter n in the coupled water-energy balance equation). This suggests that the regional long-term water balance should not vary along a single Budyko curve; instead, it should form a group of Budyko curves owing to the interactions between vegetation, climate, and water cycle. A positive correlation was found between water balance component (E/P) and vegetation coverage (M) for most of the Yellow River basin and for the Inland River basin, while a negative correlation of M $ E/P was found in the Hai River basin. Vegetation coverage was successfully incorporated into an empirical equation for estimating the catchment landscape parameter n in the coupled water-energy balance equation. It was found that interannual variability in vegetation coverage could improve the estimation of the interannual variability in regional water balance.

Anthropogenic activities have been significantly perturbing global freshwater flows and groundwater reserves. Despite numerous advances in the development of land surface models (LSMs) and global terrestrial hydrological models (GHMs), relatively few studies have attempted to simulate the impacts of anthropogenic activities on the terrestrial water cycle using the framework of LSMs. From the comparison of simulated terrestrial water storage with the Gravity Recovery and Climate Experiment (GRACE) satellite observations it is found that a process-based LSM, the Minimal Advanced Treatments of Surface Interaction and Runoff (MATSIRO), outperforms the bucket-model-based GHM called H08 in simulating hydrologic variables, particularly in water-limited regions. Therefore, the water regulation modules of H08 are incorporated into MATSIRO. Further, a new irrigation scheme based on the soil moisture deficit is developed. Incorporation of anthropogenic water regulation modules significantly improves river discharge simulation in the heavily regulated global river basins. Simulated irrigation water withdrawal for the year 2000 (2462 km 3 yr 21 ) agrees well with the estimates provided by the Food and Agriculture Organization (FAO). Results indicate that irrigation changes surface energy balance, causing a maximum increase of ;50 W m 22 in latent heat flux averaged over June-August. Moreover, unsustainable anthropogenic water use in 2000 is estimated to be ;450 km 3 yr 21 , which corresponds well with documented records of groundwater overdraft, representing an encouraging improvement over the previous modeling studies. Globally, unsustainable water use accounts for ;40% of blue water used for irrigation. The representation of anthropogenic activities in MATSIRO makes the model a suitable tool for assessing potential anthropogenic impacts on global water resources and hydrology.

Abstract. Here we analyze observed dharacteristics of the natural variability in the regional-scale hydrological cycle of Illinois, including the soil and atmospheric branches. This analysis is based on a consistent data set that describes several hydrological variables: the flux of atmospheric water vapor, incoming solar radiation, precipitation, soil moisture content, aquifer water level, and river flow. The climatology of the average regional hydrological cycle has been estimated. Variability in incoming solar radiation, not precipitation, is the main forcing of the seasonal variability in evaporation, soil moisture content, aquifer water level, and river flow. While precipitation plays a minor role in shaping the natural variability in the regional hydrological cycle at the seasonal timescale, variability in precipitation is the major factor in shaping the natural variability in the regional hydrological cycle at the interannual timescale. The anomalies in the different variables of the regional hydrological cycle have been computed and the persistence patterns of extreme floods and droughts have been compared. The 1988 drought left a signature in the aquifer water level that is significantly more persistent than the corresponding signature for the 1993 summer flood. The discharge from unconfined groundwater aquifers to streams (base flow) provides an efficient dissipation mechanism for the wet anomalies in aquifer water level. However, the nonlinear dependence of the groundwater discharge on aquifer water level (groundwater rating curve) may explain why droughts leave a significantly more persistent signature on groundwater hydrology, in comparison to the signature of floods. This nonlinearity has been attributed to the increasing degree by which the unconfined aquifers get connected to the channels network, as the aquifer water level rises leading to higher drainage density. The potential implications of these results regarding the impact on regional water resources due to'any future climate change are discussed. IntroductionUnderstanding the mechanisms of natural variability in any regional hydrological cycle is a necessary step before we can make any credible predictions about the impact of future climate change on hydrology and water resources. In this paper we study direct observations on the regional-scale hydrological cycle of Illinois with the objectives of (1) identifying the statistical patterns and physical mechanisms of natural variability in the regional cycle at the seasonal and interannual timescales and (2) investigating the mechanisms of amplification and dissipation of hydrological anomalies which define how hydrological floods and droughts propagate within the regional cycle from the atmosphere down to the groundwater aquifers, including the response of the regional aquifers to floods and droughts.The hydrological cycle of North America, including Illinois, has been the subject of several studies: those of Benton et al.