Yinsheng Zhang

Land-surface hydrological processes in the permafrost region of the eastern Tibetan Plateau

In order to examine and record the effects of permafrost on hydrological processes in the cryosphere, the hydrology of the ground surface layer was investigated in the permafrost region of the eastern Tibetan Plateau. Water budget components in the surface layer were calculated on a daily basis. Variability in the water budget components, and its causes, was investigated on both seasonal and daily bases. The results showed that coupling of the thaw– frost cycle to seasonality in precipitation is the principal control of hydrological processes in the upper 2 m of the soil. When the ground surface begins to thaw, from the surface down, the concentration of melt water within a thin surface layer leads to a rapid increase in evaporation, then a slight decrease occurs in the continuous permafrost region as the wet soil zone moves downward. An analysis of seasonal variation in the water budget components demonstrated that the dominant water cycling arises from the processes of precipitation and evaporation that are typical in this region. The freeze – thaw cycle, which affects seasonal soil moisture, water storage, evaporation, and the mobilization of water through the soil and vegetation during the summer monsoon season, is a dominant feature of the land-surface hydrology in the permafrost region of the eastern Tibetan Plateau. q 2003 Elsevier B.V. All rights reserved.

Exploring the water storage changes in the largest lake (Selin Co) over the Tibetan Plateau during 2003–2012 from a basin-wide hydrological modeling

Lake water storage change (ΔSw) is an important indicator of the hydrologic cycle and greatly influences lake expansion/shrinkage over the Tibetan Plateau (TP). Accurate estimation of ΔSw will contribute to improved understanding of lake variations in the TP. Based on a water balance, this study explored the variations of ΔSw for the Lake Selin Co (the largest closed lake on the TP) during 2003–2012 using the Water and Energy Budget-based Distributed Hydrological Model (WEB-DHM) together with two different evapotranspiration (ET) algorithms (the Penman-Monteith method and a simple sublimation estimation approach for water area in unfrozen and frozen period). The contributions of basin discharge and climate causes to the ΔSw are also quantitatively analyzed. The results showed that WEB-DHM could well reproduce daily discharge, the spatial pattern, and basin-averaged values of MODIS land surface temperature (LST) during nighttime and daytime. Compared with the ET reference values estimated from the basin-wide water balance, our ET estimates showed better performance than three global ET products in reproducing basin-averaged ET. The modeled ET at point scale matches well with short-term in situ daily measurements (RMSE = 0.82 mm/d). Lake inflows and precipitation over the water area had stronger relationships with ΔSw in the warm season and monthly scale, whereas evaporation from the water area had remarkable effects on ΔSw in the cold season. The total contribution of the three factors to ΔSw was about 90%, and accounting for 49.5%, 22.1%, and 18.3%, respectively.

Evaluating the complementary relationship of evapotranspiration in the alpine steppe of the Tibetan Plateau

The complementary relationship (CR) of evapotranspiration allows the estimation of the actual evapotranspiration rate (ETa) of the land surface using only routine meteorological data, which is of great importance in the Tibetan Plateau (TP) due to its sparse observation network. With the highest in situ automatic climate observation system in a typical semiarid alpine steppe region of the TP, the wind function of Penman was replaced by one based on the Monin-Obukhov Similarity theory for calculating the potential evapotranspiration rate (ETp); the Priestley-Taylor coefficient, α, was estimated using observations in wet days; and the slope of the saturation vapor pressure curve was evaluated at an estimate of the wet surface temperature, provided the latter was smaller than the actual air temperature. A symmetric CR was obtained between the observed daily actual and potential evapotranspiration. Local calibration of the parameter value (in this order) is key to obtaining a symmetric CR: α, wet environment air temperature (Twea), and wind function. Also, present symmetric CR contradicts previous research that used default parameter values for claiming an asymmetric CR in arid and semiarid regions of the TP. The effectiveness of estimating the daily ETa via symmetric CR was greatly improved when local calibrations were implemented. At the same time, an asymmetric CR was found between the observed daily ETa and pan evaporation rates (Epan), both for D20 aboveground and E601B sunken pans. The daily ETa could also be estimated by coupling the Epan of D20 aboveground and/or E601B sunken pan through CR. The former provided good descriptors for observed ETa, while the latter still tended to overestimate it to some extent.

Modeling actual evapotranspiration with routine meteorological variables in the data-scarce region of the Tibetan Plateau: Comparisons and implications

Quantitative estimation of actual evapotranspiration (ETa) by in situ measurements and mathematical modeling is a fundamental task for physical understanding of ETa as well as the feedback mechanisms between land and the ambient atmosphere. However, the ETa information in the Tibetan Plateau (TP) has been greatly impeded by the extremely sparse ground observation network in the region. Approaches for estimating ETa solely from routine meteorological variables are therefore important for investigating spatiotemporal variations of ETa in the data-scarce region of the TP. Motivated by this need, the complementary relationship (CR) and Penman-Monteith approaches were evaluated against in situ measurements of ETa on a daily basis in an alpine steppe region of the TP. The former includes the Nonlinear Complementary Relationship (Nonlinear-CR) as well as the Complementary Relationship Areal Evapotranspiration (CRAE) models, while the latter involves the Katerji-Perrier and the Todorovic models. Results indicate that the Nonlinear-CR, CRAE, and Katerji-Perrier models are all capable of efficiently simulating daily ETa, provided their parameter values were appropriately calibrated. The Katerji-Perrier model performed best since its site-specific parameters take the soil water status into account. The Nonlinear-CR model also performed well with the advantage of not requiring the user to choose between a symmetric and asymmetric CR. The CRAE model, even with a relatively low Nash-Sutcliffe efficiency (NSE) value, is also an acceptable approach in this data-scarce region as it does not need information of wind speed and ground surface conditions. In contrast, application of the Todorovic model was found to be inappropriate in the dry regions of the TP due to its significant overestimation of ETa as it neglects the effect of water stress on the bulk surface resistance. Sensitivity analysis of the parameter values demonstrated the relative importance of each parameter in the corresponding model. Overall, the Nonlinear-CR model is recommended in the absence of measured ETa for local calibration of the model parameter values.

Snow ablation in an open field and larch forest of the southern mountainous region of eastern Siberia

Abstract The southern mountainous taiga region of eastern Siberia is the runoff source area of the basins of the rivers Lena and Amur, where snowmelt discharge is an important hydrological process. To evaluate the effect of the sparse larch forest canopy on snow ablation and energy balance in the snow-pack, meteorological conditions and snow ablation were observed in a larch forest (LF) and an open field (OP). At the beginning of snowmelt, the snow water equivalent was 54.4 and 95.5 mm at OP and LF, respectively. The snow disappeared at LF three days later than at OP. Sublimation accounted for about 8% of snow ablation at both sites from 1 April to 5 May 2002, the snowmelt period. The energy balance of the snowpack at the two sites was dominated by the net all-wave radiation onto the snow surface. The difference in snowmelt between the sites was primarily caused by a difference in the net all-wave radiation. Snow surface albedo correlated with snow surface density for densities from 150 to 350 kg m−3 at both sites.

A Systematic Evaluation of Noah-MP in Simulating Land-Atmosphere Energy, Water, and Carbon Exchanges Over the Continental United States

National Key Research and Development Program of China [2017YFA0603101]; China Postdoctoral Science Foundation [2017LH032, 2017M620069]; National Natural Science Foundation of China [41661144025, 41430748]; NASA MAP Program [80NSSC17K0352]; University of Arizona Germinating Research Program Success: Faculty Seed Grants

Comment on “Rescaling the complementary relationship for land surface evaporation” by R. Crago et al.

The generalized complementary relationship (GCR) model of Brusaert [2015] has been widely applied to estimate land surface evapotranspiration (E) over Chinese eastern monsoon region, Loess Plateau and Australia. However, Crago et al. [2016] (henceforth C16) recently noted a deficiency in one of his boundary conditions and proposed a novel approach to improve it. The key of this approach is to determine the ratio (xmin) of the potential evapotranspiration (Epo) to the apparent potential evapotranspiration for an entirely dry surface (Epads) at which E tends to be vanishing. As seen, the physically reasonable range of xmin should be between 0 and 1. The present comment reports that the xmin in C16 may become invalid under conditions of relatively strong available energy but weak winds if Epads is calculated by the mass-transfer-based method, thereby causing unrealistic estimation of E. A more preferable way to determine Epads is still based on the traditional Penman-based equation with consideration of the characteristics of dry air in which Epads occurs.

Plateau Wetlands, an Indispensible Habitat for the Black-Necked Crane (Grus nigricollis) - A Review

As a highly productive and sensitive ecosystem, plateau wetlands provide indispensable habitats for Grus nigricollis, an endangered species of crane. However, the destruction and degeneration of plateau wetlands caused by climate change and human disturbance have influenced the habitats and biological behaviors of Grus nigricollis. Previous studies have ignored the importance of particular habitats to the continued existence of Grus nigricollis, especially with regard to extremely fragile plateau wetland systems. In this review, the importance of plateau wetlands for the breeding, overwintering and foraging of Grus nigricollis is summarized; the impact of human activities and climate change on Grus nigricollis and on plateau wetland habitats is also presented. Further monitoring and research is required to produce substantive suggestions vis-à-vis future, and better, conservation of Grus nigricollis and its habitats.

Effects of temporal scales and space mismatches on the TRMM 3B42 v7 precipitation product in a remote mountainous area

The accurate measurement ofprecipitation is essential to understanding regional hydrological processes and hydrological cycling.Quantification of precipitation over remote regions such as the Tibetan plateau (TP) is highly unreliable because of thescarcity of rain gauges.The objective of this study is to evaluate the performance of the satellite precipitation product of Tropical Rainfall Measuring Mission (TRMM) 3B42 v7 at daily, weekly, monthly and seasonalscales. Comparison between TRMM grid precipitation and point-based rain gauge precipitation was conducted using nearest neighbor (NN) and bilinear weighted interpolation (BWI) methods. The results showed that the TRMM product could not capture daily precipitation well due to some rainfall events being missed at short time scalesbut provided reasonably good precipitation data at weekly, monthly and seasonal scales. TRMM tended to underestimate the precipitation of small rainfallevents (less than 1 mm/day), while it overestimated the precipitation of large rainfall events (greater than 20 mm/day). Consequently, TRMM showedbetter performance in the summer monsoon season than in the winter season.Through comparison, it was also found that the bilinear weighted interpolation method performs better than the nearest neighbor method in TRMM precipitation extraction.

Spatiotemporal variations of actual evapotranspiration over the Lake Selin Co and surrounding small lakes (Tibetan Plateau) during 2003–2012

Actual evapotranspiration (ETa) over the Tibetan Plateau (TP) is an important component of the water cycle, and greatly influences the water budgets of the TP lake basins. Quantitative estimation of ETa within lake basins is fundamental to physically understanding ETa changes, and thus will improve the understanding of the hydrological processes and energy balance throughout the lake basins. In this study, the spatiotemporal dynamic changes of ETa within the Lake Selin Co (the TP’s largest lake) and its surrounding small lakes and land area during 2003–2012 are examined at the basin scale. This was carried out using the well-established Water and Energy Budget-based Distributed Hydrological Model (WEB-DHM) for the land area, the Penman method for the water area when unfrozen, and a simple sublimation estimation approach for the water area when frozen. The relationships between ETa changes and controlling factors are also discussed. Results indicate that the simulated land ETa from the WEB-DHM reasonably agrees with the estimated ETa values from the nonlinear complementary relationship model using appropriately calibrated parameter values at a point scale. Land ETa displayed a non-significant increase of 7.03 mm year−1, and largely depends on precipitation. For the water area, the combined effects of reduced wind speed and net radiation offset the effect of rising temperature and vapor pressure deficit, and contributed to a non-significant decrease in evaporation of 4.17 mm year−1. Sensitivity analysis shows that vapor pressure deficit and wind speed are the most sensitive variables to the changes of evaporation from the water area.