Zhenghui Xie

A new parameterization for surface and groundwater interactions and its impact on water budgets with the variable infiltration capacity (VIC) land surface model

[1] This paper describes a new parameterization to represent surface and groundwater interaction dynamics for land surface models. With the new parameterization, effects of surface and groundwater interactions on soil moisture, evapotranspiration, runoff, and recharge can be dynamically taken into account. The new parameterization is implemented into the three-layer variable infiltration capacity (VIC-3L) model, which is a hydrologically based land surface scheme. The new version of VIC (called VIC-ground) is applied to two watersheds in Pennsylvania over multiple years. Results show that VIC-ground can properly simulate the movement of the daily groundwater table over multiple years at the study sites. Preliminary comparisons of VIC simulations with and without consideration of the dynamics of surface and groundwater interactions show an important impact of such interactions on the partitioning of water budget components. In particular, soil moisture of the lower layer from the VIC-ground simulations is generally wetter than that from VIC-3L. For the top thin soil layer and the upper layer of VIC-3L, soil moisture is generally drier in VIC-ground than that in VIC-3L. Such characteristics of VIC-ground result in lower surface runoff peaks and higher base flow, as well as generally less evapotranspiration compared to VIC-3L at the two study sites. Results at both sites show that it takes 3–4 years to have the effects of the initializations of groundwater tables disappear when the groundwater table is initialized to be deeper than the observed level, while it takes much less time (e.g., about 1.5 years) if the groundwater table is initialized to be shallower than the observed level. In addition, the preliminary sensitivity studies at both sites show that there is a more significant persistent signature of the impact of the precipitation when its amount is halved (i.e., 0.5 ppt) than that when its amount is doubled (i.e., 2 ppt). INDEX TERMS: 1655 Global Change: Water cycles (1836); 1833 Hydrology: Hydroclimatology; 1836 Hydrology: Hydrologic budget (1655); 1829 Hydrology: Groundwater hydrology; 1866 Hydrology: Soil moisture; KEYWORDS: surface and groundwater interactions, groundwater table, soil moisture, evaporation, runoff, VIC land surface model Citation: Liang, X., Z. Xie, and M. Huang, A new parameterization for surface and groundwater interactions and its impact on water budgets with the variable infiltration capacity (VIC) land surface model, J. Geophys. Res., 108(D16), 8613, doi:10.1029/2002JD003090, 2003.

A new surface runoff parameterization with subgrid-scale soil heterogeneity for land surface models

Abstract Soil heterogeneity plays an important role in determining surface runoff generation mechanisms. At the spatial scales represented by land surface models used in regional climate model and/or global general circulation models (GCMs) for numerical weather prediction and climate studies, both infiltration excess (Horton) and saturation excess (Dunne) runoff may be present within a studied area or a model grid cell. Proper modeling of surface runoff is essential to a reasonable representation of feedbacks in the land–atmosphere system. In this paper, a new surface runoff parameterization that dynamically represents both Horton and Dunne runoff generation mechanisms within a model grid cell is presented. The new parameterization takes into account of effects of soil heterogeneity on Horton and Dunne runoff. A series of numerical experiments are conducted to study the effects of soil heterogeneity on Horton and Dunne runoff and on soil moisture storage under different soil and precipitation conditions. The new parameterization is implemented into the current version of the hydrologically based variable infiltration capacity (VIC) land surface model and tested over three watersheds in Pennsylvania. Results show that the new parameterization plays a very important role in partitioning the water budget between surface runoff and soil moisture in the atmosphere–land coupling system. Significant underestimation of the surface runoff and overestimation of subsurface runoff and soil moisture could be resulted if the Horton runoff mechanism were not taken into account. Also, the results show that the Horton runoff mechanism should be considered within the context of subgrid-scale spatial variability of soil properties and precipitation. An assumption of time-invariant spatial distribution of potential infiltration rate may result in large errors in surface runoff and soil moisture. In addition, the total surface runoff from the new parameterization is less sensitive to the choice of the soil moisture shape parameter of the distribution.

Regional Parameter Estimation of the VIC Land Surface Model: Methodology and Application to River Basins in China

Abstract This paper presents a methodology for regional parameter estimation of the three-layer Variable Infiltration Capacity (VIC-3L) land surface model with the goal of improving the streamflow simulation for river basins in China. This methodology is designed to obtain model parameter estimates from a limited number of calibrated basins and then regionalize them to uncalibrated basins based on climate characteristics and large river basin domains, and ultimately to continental China. Fourteen basins from different climatic zones and large river basins were chosen for model calibration. For each of these basins, seven runoff-related model parameters were calibrated using a systematic manual calibration approach. These calibrated parameters were then transferred within the climate and large river basin zones or climatic zones to the uncalibrated basins. To test the efficiency of the parameter regionalization method, a verification study was conducted on 19 independent river basins in China. Overall, the...

An ensemble-based explicit four-dimensional variational assimilation method

[1] The adjoint and tangent linear models in the traditional four-dimensional variational data assimilation (4DVAR) are difficult to obtain if the forecast model is highly nonlinear or the model physics contains parameterized discontinuities. A new method (referred to as POD-E4DVAR) is proposed in this paper by merging the Monte Carlo method and the proper orthogonal decomposition (POD) technique into 4DVAR to transform an implicit optimization problem into an explicit one. The POD method is used to efficiently approximate a forecast ensemble produced by the Monte Carlo method in a 4-dimensional (4-D) space using a set of base vectors that span the ensemble and capture its spatial structure and temporal evolution. After the analysis variables are represented by a truncated expansion of the base vectors in the 4-D space, the control (state) variables in the cost function appear explicit so that the adjoint model, which is used to derive the gradient of the cost function with respect to the control variables in the traditional 4DVAR, is no longer needed. The application of this new technique significantly simplifies the data assimilation process and retains the two main advantages of the traditional 4DVAR method. Assimilation experiments show that this ensemble-based explicit 4DVAR method performs much better than the traditional 4DVAR and ensemble Kalman filter (EnKF) method. It is also superior to another explicit 4DVAR method, especially when the forecast model is imperfect and the forecast error comes from both the noise of the initial field and the uncertainty in the forecast model. Computational costs for the new POD-E4DVAR are about twice as the traditional 4DVAR method but 5% less than the other explicit 4DVAR and much lower than the EnKF method. Another assimilation experiment conducted within the Lorenz model indicates potential wider applications of this new POD-E4DVAR method.

Effects of water table dynamics on regional climate: A case study over east Asian monsoon area

[1] Groundwater is an important component of the hydrologic cycle, and its anomaly will result in variations of soil moisture, water, and energy balances between the land surface and atmosphere, which ultimately influence climate. In this study, we implement a groundwater model into the regional climate model RegCM3, which is called RegCM3_GW, and investigate the effects of water table dynamics on regional climate. Numerical experiments by RegCM3_GW and RegCM3 over the east Asian monsoon area show that incorporating the water table dynamics into the regional climate model reduces the systematic biases of the simulated precipitation by 38.5% and 39.8% over semiarid and humid regions, respectively, and increases the bias slightly by 5.6% over semihumid regions. To seek the reasons for the differences of simulated precipitation, we analyze the atmospheric water vapor budget and the local water cycle among the water table, soil moisture, evapotranspiration (ET), and convective precipitation. It is found that the top and root zone soil layers become wetter and enhance the bare soil evaporation but do not always increase the transpiration. Because of the variations of each ET’s component, the obvious enhancements of ET occur in semiarid regions and contribute to more instable profiles of pseudoequivalent potential temperature. The atmospheric moisture budget analysis indicates that the recycling rate and precipitation efficiency increase greatly over semiarid regions, which presents a local aquifer-atmosphere feedback, while the variations of atmospheric water vapor transport control the development of precipitation over semihumid and humid regions. Therefore, the effects of water table dynamics on regional climate consist of the local aquifer-atmosphere interaction and the changes of circulation originated from ambient aquifer-atmosphere interaction, and the latter factor plays an important role in the monsoon area. Sensitivity of the results to a change in convection parameterization is also explored.

Climatic responses to anthropogenic groundwater exploitation: a case study of the Haihe River Basin, Northern China

In this study, a groundwater exploitation scheme is incorporated into the regional climate model, RegCM4, and the climatic responses to anthropogenic alteration of groundwater are then investigated over the Haihe River Basin in Northern China where groundwater resources are overexploited. The scheme models anthropogenic groundwater exploitation and water consumption, which are further divided into agricultural irrigation, industrial use and domestic use. Four 30-year on-line exploitation simulations and one control test without exploitation are conducted using the developed model with different water demands estimated from relevant socioeconomic data. The results reveal that the groundwater exploitation and water consumption cause increasing wetting and cooling effects on the local land surface and in the lower troposphere, along with a rapidly declining groundwater table in the basin. The cooling and wetting effects also extended outside the basin, especially in the regions downwind of the prevailing westerly wind, where increased precipitation occurs. The changes in the four exploitation simulations positively relate to their different water demands and are highly non-linear. The largest changes in climatic variables usually appear in spring and summer, the time of crop growth. To gain further insights into the direct changes in land-surface variables due to groundwater exploitation regardless of the atmospheric feedbacks, three off-line simulations using the land surface model Community Land Model version 3.5 are also conducted to distinguish these direct changes on the land surface of the basin. The results indicate that the direct changes of land-surface variables respond linearly to water demand if the climatic feedbacks are not considered, while non-linear climatic feedbacks enhance the differences in the on-line exploitation simulations.

A model for assessing effects of climate change on runoff in China

A model is established for assessing the effects of climate change on runoff in China based on the land surface parameterization scheme variable infiltration capacity (VIC). The entire area of China is represented by 2604 cells with a resolution of 60 km × 60 km for each cell. Forcing data, soil and vegetation parameters needed by the VIC model for the entire area of China are prepared. Daily forcing data, which are obtained from 740 stations between 1980 and 1990, are interpolated to the 60 km × 60 km grid system. The VIC model is run on every grid cell over the whole China, and a routing scheme is run offline with daily input of surface runoff and drainage from the VIC to get hydrograph at basin outlets. The spatial patterns of simulated runoff and mean annual precipitation are consistent very well. The results of monthly streamflow simulations over the Huaihe and Weihe River basins indicate that there is a good agreement between the observed and simulated values, and also initially indicate the rationality and feasibility of the evaluation model.

Severe summer heatwave and drought strongly reduced carbon uptake in Southern China

Increasing heatwave and drought events can potentially alter the carbon cycle. Few studies have investigated the impacts of hundred-year return heatwaves and droughts, as those events are rare. In the summer of 2013, southern China experienced its strongest drought and heatwave on record for the past 113 years. We show that the record-breaking heatwave and drought lasted two months (from July to August), significantly reduced the satellite-based vegetation index and gross primary production, substantially altered the regional carbon cycle, and produced the largest negative crop yield anomaly since 1960. The event resulted in a net reduction of 101.54 Tg C in carbon sequestration in the region during these two months, which was 39–53% of the annual net carbon sink of China’s terrestrial ecosystems (190–260 Tg C yr−1). Moreover, model experiments showed that heatwaves and droughts consistently decreased ecosystem vegetation primary production but had opposite impacts on ecosystem respiration (TER), with increased TER by 6.78 ± 2.15% and decreased TER by 15.34 ± 3.57% assuming only changed temperature and precipitation, respectively. In light of increasing frequency and severity of future heatwaves and droughts, our study highlights the importance of accounting for the impacts of heatwaves and droughts in assessing the carbon sequestration in terrestrial ecosystems.

An application of the VIC-3L land surface model and remote sensing data in simulating streamflow for the Hanjiang River basin

The hydrologically based three-layer variable infiltration capacity (VIC-3L) land surface model with a new surface runoff model is applied to simulate streamflow for the Hanjiang River basin in China. The required soil parameters are derived from the soil classification information of global 5 min data provided by the National Atmospheric and Oceanic Administration (NOAA) Hydrology Office, the vegetation parameters are derived based on advanced very high resolution radiometer (AVHRR) and land data assimilation system (LDAS) information, and the forcing data are obtained through interpolation methods based on 740 meteorological stations. All of the data (i.e., soil, vegetation, and forcings) required by the VIC-3L model are compiled at 25 km × 25 km resolution for the Hanjiang River basin, and the daily forcing data are available for the period of 1980-1990. The VIC-3L model is applied to the Hanjiang River basin, and the VIC-3L simulated daily runoff is routed to the outlets of six streamflow stations and compared with the daily and monthly observed streamflow at the stations. The results show that the model can simulate the observations well.

Increasing flash droughts over China during the recent global warming hiatus

The recent global warming slowdown or hiatus after the big El Niño event in 1997/98 raises the questions of whether terrestrial hydrological cycle is being decelerated and how do the hydrological extremes respond to the hiatus. However, the rapidly developing drought events that are termed as “flash droughts” accompanied by extreme heat, low soil moisture and high evapotranspiration (ET), occurred frequently around the world, and caused devastating impacts on crop yields and water supply. Here, we investigate the long-term trend and variability of flash droughts over China. Flash droughts are most likely to occur over humid and semi-humid regions, such as southern and northeastern China. Flash drought averaged over China increased by 109% from 1979 to 2010, and the increase was mainly due to a long term warming of temperature (50%), followed by the contributions from decreasing soil moisture and increasing ET. There was a slight drop in temperature after 1997, but the increasing trend of flash droughts was tripled. Further results indicate that the decreasing temperature was compensated by the accelerated drying trends of soil moisture and enhanced ET, leading to an acceleration of flash droughts during the warming hiatus. The anthropogenic warming in the next few decades may exacerbate future flash drought conditions in China.