Impacts of climate change on snow accumulation and melting processes over mountainous regions in Northern California during the 21st century.

A point-location-based analysis of future climate change impacts on snow accumulation and melting processes was conducted over three study watersheds in Northern California during a 90-year future period by means of snow regime projections. The snow regime projections were obtained by means of a physically-based snow model with dynamically downscaled future climate projections. Then, atmospheric and snow-related variables, and their interrelations during the 21st century were investigated to reveal future climate change impacts on snow accumulation and melting processes. The analysis shows large reductions in snow water equivalent (SWE), snowfall to precipitation (S/P) ratio, and snowmelt through the 21st century. Timing of the peak of the SWE and snowmelt will also change in the future. Meanwhile, the analysis in this study shows that air temperature rise will affect, but will not dominate the future change in snowmelt over the study watersheds. This result implies the importance of considering atmospheric variables other than air temperature, such as precipitation, shortwave radiation, relative humidity, and wind speed even if these variables will not clearly change during the 21st century.

Reconstruction and evaluation of changes in hydrologic conditions over a transboundary region by a regional climate model coupled with a physically-based hydrology model: Application to Thao river watershed.

The differences among countries in terms of physical features, governmental policies, priorities in short- and long-term water resources management may lead to conflicts in managing and sharing of water resources over the transboundary regions. Due to no formal data sharing agreement between countries, there has been usually no data availability at transboundary regions. In this study, a methodology, in which a physically-based hydrology model was coupled with a regional climate model, is proposed to reconstruct and evaluate hydrologic conditions over transboundary regions. For the case study, Thao river watershed (TRW), within Vietnam and China, was selected. The Watershed Environmental Hydrology (WEHY) model was implemented based on topography, soil, and land use/cover information which was retrieved from global satellite data resources. The watershed model-WEHY was first validated over the TRW, and then was used to reconstruct historical hydrologic conditions during 1950-2007. The results of this study suggest no significant trend in the annual streamflow over the target watershed. In addition, there is a time shift in the wet season between the upstream sector in China and the downstream sector in Vietnam over the TRW. The annual flow contribution from the upstream sector in China to the outlet of TRW is estimated to be around 66%, and the remaining 34% contribution comes from the downstream sector in Vietnam territory. Last but not the least, the annual flow as a function of return period varies not only with the return period but also as a function of the time window, reflecting the effect of the changing regime on the streamflows at the TRW. The evolution of the flow frequency through time is an evidence of the ongoing non-stationarity in the hydrologic conditions over TRW.

Analysis of future climate change impacts on snow distribution over mountainous watersheds in Northern California by means of a physically-based snow distribution model.

The impacts of climate change on snow distribution through the 21st century were investigated over three mountainous watersheds in Northern California by means of a physically-based snow distribution model. The future climate conditions during a 90-year future period from water year 2010 to 2100 were obtained from 13 future climate projection realizations from two GCMs (ECHAM5 and CCSM3) based on four SRES scenarios (A1B, A1FI, A2, and B1). The 13 future climate projection realizations were dynamically downscaled at 9 km resolution by a regional climate model. Using the downscaled variables based on the 13 future climate projection realizations, snow distribution over the Feather, Yuba, and American River watersheds (FRW, YRW, and ARW) was projected by means of the physically-based snow model. FRW and YRW watersheds cover the main source areas of the California State Water Project (SWP), and ARW is one of the key watersheds in the California Central Valley Project (CVP). SWP and CVP are of great importance as they provide and regulate much of the California's water for drinking, irrigation, flood control, environmental, and hydro-power generation purposes. Ensemble average snow distribution over the study watersheds was calculated over the 13 realizations and for each scenario, revealing differences among the scenarios. While the snow reduction through the 21st century was similar between A1B and A2, the snow reduction was milder for B1, and more severe for A1FI. A significant downward trend was detected in the snowpack over nearly the entire watershed areas for all the ensemble average results.

Integrating global land-cover and soil datasets to update saturated hydraulic conductivity parameterization in hydrologic modeling.

Soil properties play an important role in watershed hydrology and environmental modeling. In order to model realistic hydrologic processes, it is necessary to obtain compatible soil data. This study introduces a new method that integrates global soil databases with land use/land cover (LULC) databases to better represent saturated hydraulic conductivity (Ks) which is one of the most important soil properties in hydrologic modeling. The Ks is modified by means of uniting physical infiltration mechanisms with hydrologic soil-LULC complexes from lookup tables from USDA-SCS (1985). This approach enables assimilation of available coarse resolution soil parameters by the finer resolution global LULC datasets. In order to test the performance of the proposed approach, it has been incorporated into the Watershed Environmental Hydrology (WEHY) model to simulate hydrologic conditions over the Cache Creek Watershed (CCW) and Shasta Dam Watershed (SDW) in Northern California by means of different soil datasets. Soil dataset S1 was obtained from the local soil database including SSURGO (Web soil survey, USDA). The second soil dataset (S2) is the global ISRIC soil data SoilGrids-1km obtained from World Soil Information. Soil dataset S4 is global FAO soil data. The third (S3) and fifth (S5) soil datasets were calculated by integrating the LULC into global soil datasets (S2, S4), respectively. The results of this study suggest that the proposed approach can provide a fine resolution soil dataset through integration of LULC and soil data, which can improve the estimation of soil hydraulic parameters and the performance of hydrologic modeling over the target watersheds. Within this framework, the new approach of this study can be applied widely in many parts of the world by means of the global soil and LULC databases.

Trend analysis of watershed-scale precipitation over Northern California by means of dynamically-downscaled CMIP5 future climate projections.

The impacts of climate change on watershed-scale precipitation through the 21st century were investigated over eight study watersheds in Northern California based on dynamically downscaled CMIP5 future climate projections from three GCMs (CCSM4, HadGEM2-ES, and MIROC5) under the RCP4.5 and RCP8.5 future climate scenarios. After evaluating the modeling capability of the WRF model, the six future climate projections were dynamically downscaled by means of the WRF model over Northern California at 9km grid resolution and hourly temporal resolution during a 94-year period (2006-2100). The biases in the model simulations were corrected, and basin-average precipitation over the eight study watersheds was calculated from the dynamically downscaled precipitation data. Based on the dynamically downscaled basin-average precipitation, trends in annual depth and annual peaks of basin-average precipitation during the 21st century were analyzed over the eight study watersheds. The analyses in this study indicate that there may be differences between trends of annual depths and annual peaks of watershed-scale precipitation during the 21st century. Furthermore, trends in watershed-scale precipitation under future climate conditions may be different for different watersheds depending on their location and topography even if they are in the same region.

Assessment of 21st century drought conditions at Shasta Dam based on dynamically projected water supply conditions by a regional climate model coupled with a physically-based hydrology model.

Along with socioeconomic developments, and population increase, natural disasters around the world have recently increased the awareness of harmful impacts they cause. Among natural disasters, drought is of great interest to scientists due to the extraordinary diversity of their severity and duration. Motivated by the development of a potential approach to investigate future possible droughts in a probabilistic framework based on climate change projections, a methodology to consider thirteen future climate projections based on four emission scenarios to characterize droughts is presented. The proposed approach uses a regional climate model coupled with a physically-based hydrology model (Watershed Environmental Hydrology Hydro-Climate Model; WEHY-HCM) to generate thirteen equally likely future water supply projections. The water supply projections were compared to the current water demand for the detection of drought events and estimation of drought properties. The procedure was applied to Shasta Dam watershed to analyze drought conditions at the watershed outlet, Shasta Dam. The results suggest an increasing water scarcity at Shasta Dam with more severe and longer future drought events in some future scenarios. An important advantage of the proposed approach to the probabilistic analysis of future droughts is that it provides the drought properties of the 100-year and 200-year return periods without resorting to any extrapolation of the frequency curve.

Future climate change impact assessment of watershed scale hydrologic processes in Peninsular Malaysia by a regional climate model coupled with a physically-based hydrology modelo.

Impacts of climate change on the hydrologic processes under future climate change conditions were assessed over Muda and Dungun watersheds of Peninsular Malaysia by means of a coupled regional climate and physically-based hydrology model utilizing an ensemble of future climate change projections. An ensemble of 15 different future climate realizations from coarse resolution global climate models' (GCMs) projections for the 21st century was dynamically downscaled to 6km resolution over Peninsular Malaysia by a regional climate model, which was then coupled with the watershed hydrology model WEHY through the atmospheric boundary layer over Muda and Dungun watersheds. Hydrologic simulations were carried out at hourly increments and at hillslope-scale in order to assess the impacts of climate change on the water balances and flooding conditions in the 21st century. The coupled regional climate and hydrology model was simulated for a duration of 90years for each of the 15 realizations. It is demonstrated that the increase in mean monthly flows due to the impact of expected climate change during 2040-2100 is statistically significant from April to May and from July to October at Muda watershed. Also, the increase in mean monthly flows is shown to be significant in November during 2030-2070 and from November to December during 2070-2100 at Dungun watershed. In other words, the impact of the expected climate change will be significant during the northeast and southwest monsoon seasons at Muda watershed and during the northeast monsoon season at Dungun watershed. Furthermore, the flood frequency analyses for both watersheds indicated an overall increasing trend in the second half of the 21st century.

Evaluation of overland flow model for a hillslope using laboratory flume data.

Comprehensive modelling of overland flow requires models for both rill and interrill area overland flow. Evaluation of a physically based mathematical model for simulating overland flow generated on rill and interrill areas of hillslope was done using a data set gathered from a laboratory experimental setup. A rainfall simulator has been constructed together with a 6.50 m × 1.36 m erosion flume that can be given adjustable slopes in both longitudinal and lateral directions. The model was calibrated and validated using the experimental results from the setup of the flume having 5% lateral and 10% longitudinal slopes where rainfall intensities of 105 and 45 mm/hr were induced with the use of nozzles. Results show that for the given slope combination, the model was capable of simulating the flow coming from the rill and interrill areas for the two different rainfall intensities. It was found that significantly more of the flow occurred in the form of the rill flow. The model studied here can be used for the better prediction of overland flow and can also be used as a building block for an associated erosion and sediment transport model.