Impacts of forest changes on hydrology: a case study of large watersheds in the upper reaches of Minjiang River watershed in China

Quantifying the effects of forest changes on hydrology in large watersheds is important for designing forest or land management and adaptation strategies for watershed ecosystem sustainability. Minjiang River watershed, located in the upper reach of the Yangtze River basin, plays a strategic role in the environmental protection and economic and social well-being for both the watershed and the entire Yangtze River basin. The watershed lies in the transition zone from Sichuan Basin to Qinghai-Tibet Plateau with a size of 24 000 km<sup>2</sup>. Due to its strategic significance, severe historic deforestation and high sensitivity to climate change, the watershed has long been recognized as one of the highest priority watersheds in China for scientific research and resource management. The purpose of this review paper is to provide a state-of-the-art summary on what we have learned from several recently completed research programs (one of them known as "973 of the China National Major Fundamental Science" from 2002 to 2008). This summary paper focused on how land cover or forest change affected hydrology at both forest stand and watershed scales in this large watershed. Inclusion of two different spatial scales is useful, because the results from a small spatial scale (e.g. forest stand level) can help interpret the findings on a large spatial scale. Our review suggests that historic forest harvesting or land cover change has caused significant water yield increase due to reduction of forest canopy interception and evapotranspiration caused by removal of forest vegetation on both spatial scales. The impact magnitude caused by forest harvesting indicates that the hydrological effects of forest or land cover changes can be as important as those caused by climate change, while the opposite impact directions suggest their offsetting effects on water yield in the Minjiang River watershed. In addition, different types of forests have different magnitudes of evapotranspiration (ET), with the lowest in old-growth natural coniferous forests (<i>Abies faxoniana</i> Rehd. et Wils.) and the highest in coniferous plantations (e.g. <i>Picea asperata</i> Mast.) among major forest types in the study watershed. This suggests that selection of different types of forests can have an important role in ET and consequently water yield. Our synthesis indicates that future reforestation and climate change would likely produce the hydrological effects in the same direction and thus place double the pressure on water resource as both key drivers may lead to water yield reduction. The findings can support designing management strategies for protection of watershed ecological functions in the context of future land cover and climate changes

Effects of vegetation patterns on yields of the surface and subsurface waters in the Heishui Alpine Valley in west China

International audienceThe relationships between different vegetation types and water yields were investigated in the Heishui Valley of the upper Yangtze River in western China. Contributions of groundwater and the water from surface and subsurface in different tributaries were, respectively, computed based on the stable isotope data, while the percentages of different vegetation covers were achieved by remote sensing in landscape scale. Based on the relationships between different vegetation types and water yields in seven watersheds, we found that reduction in the total vegetation, forest and subalpine coniferous forest covers could cause increasing in surface and subsurface water yields, while the water yield increased with the alpine shrub and meadow cover increasing, respectively. All the relationships were displayed as the low altitude and high altitude patterns, which were caused by the different vegetation characteristics and topography. We also found effects of the total vegetation cover played the most important role on water yield at large scale while the coniferous forest cover would affect the water yield at relatively small scale

Data-driven Analysis of Potential Impacts of Land-use/cover Change on Water Resources in Coastal Watersheds: Perspectives from Non-stationarity and Nonlinearity

Water resource conditions are highly influenced by human activities. As one of the most important indicators that reflects the intensity of human activities, LUCC has drawn much attention in recent decades. Thus, it is necessary to understand the LUCC patterns in watersheds and identify their impacts on the local water resources. We also analyzed the impacts of the human activities on the streamflow regime as well as the regional climate changes. Furthermore, the nonlinear relationship between land use and water quality was identified in this study. The major findings of this study are as follows: (1) Spatial variation in land use was highly related to the driving factors, and population and local economic development may be the major factors influencing urbanization processes in the coastal watersheds. (2) Streamflow extremes are highly impacted by the human activities and climate variability, and the human activities may be the major factor controlling streamflow extremes at short time scales. (3) The coupled effects of climate variability and human activities were identified by analyzing the relationship between urbanization and climate patterns in the studied watersheds, and the patterns of precipitation and temperature may be modified in highly urbanized areas. (4) A nonlinear relationship between land use and water quality has been widely observed, especially in highly polluted watersheds

Hydrology in Water Resources Management

This book is a collection of 12 papers describing the role of hydrology in water resources management. The papers can be divided s according to their area of focus as 1) modeling of hydrological processes, 2) use of modern techniques in hydrological analysis, 3) impact of human pressure and climate change on water resources, and 4) hydrometeorological extremes. Belonging to the first area is the presentation of a new Muskingum flood routing model, a new tool to perform frequency analysis of maximum precipitation of a specified duration via the so-named PMAXΤP model (Precipitation MAXimum Time (duration) Probability), modeling of interception processes, and using a rainfall-runoff GR2M model to calculate monthly runoff. For the second area, the groundwater potential was evaluated using a model of multi-influencing factors in which the parameters were optimized by using geoprocessing tools in geographical information system (GIS) in combination with satellite altimeter data and the reanalysis of hydrological data to simulate overflow transport using the Nordic Sea as an example. Presented for the third area are a water balance model for the comparison of water resources with the needs of water users, the idea of adaptive water management, impacts of climate change, and anthropogenic activities on the runoff in catchment located in the western Himalayas of Pakistan. The last area includes spatiotemporal analysis of rainfall variability with regard to drought hazard and use of the copula function to meteorologically analyze drought

Effects of Mountain Rivers Cascade Hydropower Stations on Water Ecosystems

China is rich in hydropower resources, and mountain rivers have abundant water resources and huge development potential, which have a profound impact on the pattern of water resources allocation in China. As the main way of water resources and hydropower development, the construction of cascade hydropower stations, while meeting the requirements of water resources utilization for social development, has also brought adverse effects on river ecosystems. Therefore, the impact of the construction of cascade hydropower stations on mountainous river ecosystems, where the minimum ecological flow of rivers must be ensured and reviewed. In addition, this paper proposed the deficiencies and outlooks for cascade hydropower stations based on previous research results

Effect of the Substitution of Agricultural Uses by Forest on the Hydrological Processes in a Tropical Watershed. Analysis through Hydrological Simulation

Forests play an important role in watershed hydrology, regulating the transfer of water within the system. Their role in maintaining the hydrological regime of watersheds is still a controversial issue. Due to the uncertainty, our first objective was to identify, through GIS techniques, “Environmentally Sensitive Areas” (ESAs) in the Pinhal watershed, subsequently, to simulate land use scenarios with the Soil and Water Assessment Tool model. In one of these scenarios, we considered these areas as protected by forest cover. This scenario we compared to the current scenario regarding watershed sediment yield and hydrological processes. The results showed a reduction in sediment yield of 54% between the two scenarios, whereas watershed water yield was reduced by 19.3%