Peihua Qin

Effects of anthropogenic water regulation and groundwater lateral flow on land processes

Both anthropogenic water regulation and groundwater lateral flow essentially affect groundwater table patterns. Their relationship is close because lateral flow recharges the groundwater depletion cone, which is induced by over-exploitation. In this study, schemes describing groundwater lateral flow and human water regulation were developed and incorporated into the Community Land Model 4.5. To investigate the effects of human water regulation and groundwater lateral flow on land processes as well as the relationship between the two processes, three simulations using the model were conducted for the years 2003–2013 over the Heihe River Basin in northwestern China. Simulations showed that groundwater lateral flow driven by changes in water heads can essentially change the groundwater table pattern with the deeper water table appearing in the hillslope regions and shallower water table appearing in valley bottom regions and plains. Over the last decade, anthropogenic groundwater exploitation deepened the water table by approximately 2 m in the middle reaches of the Heihe River Basin and rapidly reduced the terrestrial water storage, while irrigation increased soil moisture by approximately 0.1 m3 m−3. The water stored in the mainstream of the Heihe River was also reduced by human surface water withdrawal. The latent heat flux was increased by 30 W m−2 over the irrigated region, with an identical decrease in sensible heat flux. The simulated groundwater lateral flow was shown to effectively recharge the groundwater depletion cone caused by over-exploitation. The offset rate is higher in plains than mountainous regions.

Incorporating groundwater dynamics and surface/subsurface runoff mechanisms in regional climate modeling over river basins in China

To improve the capability of numerical modeling of climate-groundwater interactions, a groundwater component and new surface/subsurface runoff schemes were incorporated into the regional climate model RegCM3, renamed RegCM3_Hydro. 20-year simulations from both models were used to investigate the effects of groundwater dynamics and surface/subsurface runoff parameterizations on regional climate over seven river basins in China. A comparison of results shows that RegCM3_Hydro reduced the positive biases of annual and summer (June, July, August) precipitation over six river basins, while it slightly increased the bias over the Huaihe River Basin in eastern China. RegCM3_Hydro also reduced the cold bias of surface air temperature from RegCM3 across years, especially for the Haihe and the Huaihe river basins, with significant bias reductions of 0.80°C and 0.88°C, respectively. The spatial distribution and seasonal variations of water table depth were also well captured. With the new surface and subsurface runoff schemes, RegCM3_Hydro increased annual surface runoff by 0.11–0.62 mm d−1 over the seven basins. Though previous studies found that incorporating a groundwater component tends to increase soil moisture due to the consideration of upward groundwater recharge, our present work shows that the modified runoff schemes cause less infiltration, which outweigh the recharge from groundwater and result in drier soil, and consequently cause less latent heat and more sensible heat over most of the basins.

Climatic impacts of the Middle Route of the South-to-North Water Transfer Project over the Haihe River basin in North China simulated by a regional climate model

The Middle Route of the South-to-North Water Transfer Project (MSWTP) was constructed to ease the water crisis over the North China Plain. In this study, we incorporated a water transfer scheme into the regional climate model RegCM4 and investigated the climatic impacts of the MSWTP over the Haihe River Basin in North China. Four 10 year simulation tests were conducted from 2001 to 2010 where different volumes of water were transferred. The results demonstrated that before the MSWTP was conducted the original groundwater exploitation and consumption over the Haihe River Basin led to wetting and cooling at the land surface with rapidly falling groundwater depth. The extra water input from the MSWTP slightly enhanced the wetting and cooling effects over the basin, as well as reduced the falling rate in the groundwater depth along the conveyance line. However, the weak climatic effects of the MSWTP were limited at a local scale and had no obvious interannual trends, because the transfer volume of the MSWTP was far lower than the total demand which has been conventionally satisfied through local water exploitation. In terms of seasonal variations, the greatest changes due to the MSWTP occurred in the summer for precipitation and soil moisture and in the spring for energy-related variables (heat fluxes and 2 m air temperature).

A soil water and heat transfer model including changes in soil frost and thaw fronts

Freeze-thaw processes in soils, including changes in frost and thaw fronts (FTFs), are important physical processes. The movement of FTFs affects soil hydrothermal characteristics, as well as energy and water exchanges between the land surface and the atmosphere and hydrothermal processes in the land surface. This paper reduces the issue of soil freezing and thawing to a multiple moving-boundary problem and develops a soil water and heat transfer model which considers the effects of FTF on soil hydrothermal processes. A local adaptive variable-grid method is used to discretize the model. Sensitivity tests based on the hierarchical structure of the Community Land Model (CLM) show that multiple FTFs can be continuously tracked, which overcomes the difficulties of isotherms that cannot simultaneously simulate multiple FTFs in the same soil layer. The local adaptive variable-grid method is stable and offers computational efficiency several times greater than the high-resolution case. The simulated FTF depths, soil temperatures, and soil moisture values fit well with the observed data, which further demonstrates the potential application of this simulation to the land-surface process model.

Precipitation extremes in the dry and wet regions of China and their connections with the sea surface temperature in the eastern tropical Pacific Ocean

We investigated the connections between the precipitation extremes during 1953–2002 in the dry and wet regions of China and the sea surface temperature (SST) in the eastern tropical Pacific Ocean (ETP; including EI Nino-Southern Oscillation 1.2, 3, and 3.4 regions) based on two sets of observations, 17 CMIP5 models, and nine regional climate model (RCMs) results, which were downscaled using the RCM RegCM4. The grid cells with the lowest 30% non-missing precipitation extremes were identified as the dry region and those with the highest 30% comprised the wet region. Compared with observed extreme indices, the CMIP5 ensemble could simulate the temporal averages and spatial patterns of extreme indices in the dry and wet regions, where the temporal averages of the indices based on RCMs matched better with those of the observed indices. In the dry region of China, the extreme precipitation indices had positive regression coefficients versus the SST of the ETP for all data sets, but the linear relationships of the extreme indices with the SST were more complex in the wet region. Finally, we investigated the variations of the correlations of precipitation extremes with the SST of ETP in multiple RCM results and CMIP5 models, respectively.

Land Surface Improvements

Land surface processes play an important role in the climate system. In this chapter, some improvements to parameterizations of land surface processes are introduced, including representation of water table dynamics, inclusion of anthropogenic groundwater exploitation, development of a frozen soil model considering the freezing–thawing interface, representation of crop growth, and development of a land surface model. Furthermore, the development of our land data assimilation system will be also described in this chapter.