Dawen Yang

Impact of vegetation coverage on regional water balance in the nonhumid regions of China.

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 approximately 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.

The hysteretic evapotranspiration—Vapor pressure deficit relation

Diurnal hysteresis between evapotranspiration (ET) and vapor pressure deficit (VPD) was reported in many ecosystems, but justification for its onset and magnitude remains incomplete with biotic and abiotic factors invoked as possible explanations. To place these explanations within a holistic framework, the occurrence of hysteresis was theoretically assessed along a hierarchy of model systems where both abiotic and biotic components are sequentially added. Lysimeter evaporation (E) measurements and model calculations using the Penman equation were used to investigate the effect of the time lag between net radiation and VPD on the hysteresis in the absence of any biotic effects. Modulations from biotic effects on the ET-VPD hysteresis were then added using soil-plant-atmosphere models of different complexities applied to a grassland ecosystem. The results suggest that the hysteresis magnitude depends on the radiation-VPD lag, while the plant and soil water potentials are both key factors modulating the hysteretic ET-VPD relation as soil moisture declines. In particular, larger hysteresis magnitude is achieved at less negative leaf water potential, root water potential, and soil water potential. While plant hydraulic capacitance affects the leaf water potential-ET relation, it has negligible effects on the ET-VPD hysteresis. Therefore, the genesis and magnitude of the ET-VPD hysteresis are controlled directly by both abiotic factors such as soil water availability, biotic factors (leaf and root water potentials, which in turn depend on soil moisture), and the time lag between radiation and VPD.

Multiscale Hydrologic Applications of the Latest Satellite Precipitation Products in the Yangtze River Basin using a Distributed Hydrologic Model

AbstractThe present study aims to evaluate three global satellite precipitation products [TMPA 3B42, version 7 (3B42 V7); TMPA 3B42 real time (3B42 RT); and Climate Prediction Center morphing technique (CMORPH)] during 2003–12 for multiscale hydrologic applications—including annual water budgeting, monthly and daily streamflow simulation, and extreme flood modeling—via a distributed hydrological model in the Yangtze River basin. The comparison shows that the 3B42 V7 data generally have a better performance in annual water budgeting and monthly streamflow simulation, but this superiority is not guaranteed for daily simulation, especially for flood monitoring. It is also found that, for annual water budgeting, the positive (negative) bias of the 3B42 RT (CMORPH) estimate is mainly propagated into the simulated runoff, and simulated evapotranspiration tends to be more sensitive to negative bias. Regarding streamflow simulation, both near-real-time products show a region-dependent bias: 3B42 RT tends to overe...

Quantifying the effect of vegetation change on the regional water balance within the Budyko framework

The Budyko framework is widely used to investigate the impacts of climate and landscape changes on regional hydrology, but quantifying the effect of vegetation change is still a challenge due to the lack of an explicit expression of vegetation in Budyko equations. This study establishes a relationship between the change in the landscape parameter in a Budyko equation and vegetation change (represented by fPAR, the fraction of Photosynthetically Active Radiation absorbed by vegetation) for catchments in China, where, due to large-scale soil and water conservation projects implemented by the Chinese government, vegetation and regional hydrology have changed substantially over the past 30 years. The ratio of landscape parameter change to the change in fPAR has a strong relationship with the aridity index, and thus, vegetation change can be converted into a change in the landscape parameter. Then, the fPAR elasticity of runoff is introduced and formulated under the Budyko framework. It provides a useful tool for the quantitative evaluation of the regional hydrological response to vegetation change, but the proposed relationship still needs to be evaluated in other catchments around the globe where large-scale afforestation or vegetation recovery has occurred.

A distributed scheme developed for eco-hydrological modeling in the upper Heihe River

Modeling the hydrological processes at catchment scale requires a flexible distributed scheme to represent the catchment topography, river network and vegetation pattern. This study has developed a distributed scheme for eco-hydrological simulation in the upper Heihe River. Based on a 1 km × 1 km grid system, the study catchment is divided into 461 sub-catchments, whose main streams form the streamflow pathway. Furthermore, a 1 km grid is represented by a number of topographically similar “hillslope-valley” systems, and the hillslope is the basic unit of the eco-hydrological simulation. This model is tested with a simplified hydrological simulation focusing on soil-water dynamics and streamflow routing. Based on a 12-year simulation from 2001 to 2012, it is found that variability in hydrological behavior is closely associated with climatic and landscape conditions especially vegetation types. The subsurface and groundwater flows dominate the total river runoff. This implies that the soil freezing and thawing process would significantly influence the runoff generation in the upper Heihe basin. Furthermore, the runoff components and water balance characteristics vary among different vegetation types, showing the importance of coupling the vegetation pattern into catchment hydrological simulation. This paper also discusses the model improvement to be done in future study.

Optimization of Water Resources Utilization by PSO-GA

The objective of this paper is to present an optimal model to address the water resources utilization of the Tao River basin in China. The Tao River water diversion project has been proposed to alleviate the problem of water shortages in Gansu Province in China. A multi reservoir system is under consideration with multiple objectives including water diversion, ecological water demand, irrigation, hydropower generation, industrial requirements, and domestic uses in the Tao River basin. A multi-objective model for the minimization of water shortages and the maximization of hydro-power production is proposed to manage the utilization of Tao River water resources. An adjustable PSO-GA (particle swarm optimization – genetic algorithm) hybrid algorithm is proposed that combines the strengths of PSO and GA to balance natural selection and good knowledge sharing to enable a robust and efficient search of the solution space. Two driving parameters are used in the adjustable hybrid model to optimize the performance of the PSO-GA hybrid algorithm by assigning a preference to either PSO or GA. The results show that the proposed hybrid algorithm can simultaneously obtain a promising solution and speed up the convergence.

An error analysis of the Budyko hypothesis for assessing the contribution of climate change to runoff

Many previous studies have evaluated the hydrologic response to climate change using the first-order approximation (first-order Taylor expansion) of the Mezentsev-Choudhury-Yang equation (formulating the Budyko hypothesis), which has a parameter n representing catchment characteristics. However, no studies have paid attention to the error due to the first-order approximation. This study therefore estimates this error to improve the theoretical framework for assessing the contribution of climate change to runoff based on the Budyko hypothesis. Specifically, the error increases when precipitation (P) decreases and potential evaporation (E0) increases, and n increases. Therefore, an increasing P or decreasing E0 leads to an underestimate of the climatic contributions, while a decreasing P or increasing E0 leads to an overestimate. In addition, we suggest a new method to accurately estimate the contribution of climate change to runoff.

The hysteresis response of soil CO2 concentration and soil respiration to soil temperature

Diurnal hysteresis between soil temperature (Ts) and both CO2 concentration ([CO2]) and soil respiration rate (Rs) were reported across different field experiments. However, the causes of these hysteresis patterns remain a subject of debate, with biotic and abiotic factors both invoked as explanations. To address these issues, a CO2 gas transport model is developed by combining a layer-wise mass conservation equation for subsurface gas phase CO2, Fickian diffusion for gas transfer, and a CO2 source term that depends on soil temperature, moisture, and photosynthetic rate. Using this model, a hierarchy of numerical experiments were employed to disentangle the causes of the hysteretic [CO2]-Ts and CO2 flux Ts (i.e., F-Ts) relations. Model results show that gas transport alone can introduce both [CO2]-Ts and F-Ts hystereses and also confirm prior findings that heat flow in soils lead to [CO2] and F being out of phase with Ts, thereby providing another reason for the occurrence of both hystereses. The area (Ahys) of the [CO2]-Ts hysteresis near the surface increases, while the Ahys of the Rs-Ts hysteresis decreases as soils become wetter. Moreover, a time-lagged carbon input from photosynthesis deformed the [CO2]-Ts and Rs-Ts patterns, causing a change in the loop direction from counterclockwise to clockwise with decreasing time lag. An asymmetric 8-shaped pattern emerged as the transition state between the two loop directions. Tracing the pattern and direction of the hysteretic [CO2]-Ts and Rs-Ts relations can provide new ways to fingerprint the effects of photosynthesis stimulation on soil microbial activity and detect time lags between rhizospheric respiration and photosynthesis.

Multi-scale evaluation of six high-resolution satellite monthly rainfall estimates over a humid region in China with dense rain gauges

This study evaluates and compares the performance of six high-resolution monthly satellite rainfall estimates (SREs), which include TRMM 3B43V6, TRMM 3B42RTV6, CMORPH, GSMaP MWR+, GSMaP MVK+, and PERSIANN, with dense ground rain gauges located in Ganjiang River Basin. The performance was evaluated at multiple spatial scales: the 0.25° × 0.25° grid, sub-catchment, and the whole basin. It was observed that 3B43V6 generally performed well and was able to capture the ground benchmark rainfall with slight overestimation, whereas all of the other SREs suffered large underestimation in the study area. Among the five pure satellite-derived products, 3B42RTV6 and CMORPH performed better, whereas PERSIANN performed the worst. All of the SREs except 3B43V6 showed a strong seasonal signature with much better performance in the wet season than in the dry season. The results also indicate that SREs performed better in the southeast and central regions, whereas poor performance was observed in the western mountains and in the northern plains. Furthermore, the spatial patterns of SREs errors are influenced mainly by the local terrain. The performance of SREs improved when the spatial scale was increased, whereas the performance reduced when the temporal scale was increased from month to year.

Optimal Dam Operation during Flood Season Using a Distributed Hydrological Model and a Heuristic Algorithm

A physically based distributed hydrological model is coupled with an optimization algorithm for joint dam operation to reduce the flood peaks downstream. The decision variables are the release-inflow ratios. The heuristic algorithm seeds different release scenarios attempting to find the most suitable combination. The objective is to reduce the flood peak downstream, and the objective function is to minimize the difference between the simulated and threshold discharges. The latter depends on the purpose of flood management at the basin. Here, it is proposed as the mean discharge during heavy rainfall and is used to start a special dam operation. In order to fulfill the objective function, the reservoirs are expected to release water before the flood event takes place and close the gates during the flood peaks to replenish the released water beforehand. The developed system was applied to the upper Tone River in Japan where the optimal release schedule from two key dams was obtained. The observed weather radar products were input to the hydrological model to simulate the discharge within the river network. Then, the simulated inflows were input to the dam storage functions. The release is routed downstream and the river discharge is evaluated at the control point. The results indicate that the proposed integrated operation can effectively reduce a flood peak suggesting the feasibility of real-time operation in future developments.