Using Remote Sensing Techniques to Improve Hydrological Predictions in a Rapidly Changing World

Remotely sensed geophysical datasets are being produced at increasingly fast rates to monitor various aspects of the Earth system in a rapidly changing world. The efficient and innovative use of these datasets to understand hydrological processes in various climatic and vegetation regimes under anthropogenic impacts has become an important challenge, but with a wide range of research opportunities. The ten contributions in this Special Issue have addressed the following four research topics: (1) Evapotranspiration estimation; (2) rainfall monitoring and prediction; (3) flood simulations and predictions; and (4) monitoring of ecohydrological processes using remote sensing techniques. Moreover, the authors have provided broader discussions, on how to make the most out of the state-of-the-art remote sensing techniques to improve hydrological model simulations and predictions, to enhance their skills in reproducing processes for the fast-changing world.

Surface-groundwater interactions in karst: overview, concept and mapping

<p>Under special meteorological and hydrological circumstances, shallow karst areas and karst poljes may experience surface water overflow. As a result, surface- groundwater interaction occurs usually creating intermittent lakes. Although human settlements and activities have traditionally adapted to these natural conditions, extensive temporary floods are classified among the most common natural disasters in karst. On the other hand, intermittent lakes are considered as natural reservoirs of excess recharge and good flood regulators in the lower parts of river basins. Due to specific ecohydrological processes and environmental conditions, these areas host unique wetland ecosystems with high levels of biodiversity and provide various ecosystem services, such as ecological productivity, photosynthesis and carbon storage. Given the dynamic nature of hydrological processes in karst aquifers, a distinctive feature of the phenomena described is its high variability of occurrence and duration. Therefore, the identification, characterization as well as the determination of the spatial dimension of flood levels is a challenging task. Focusing on the Slovenian karst, a literature review and analysis of topographical and hydrological data of the selected study areas was conducted. The hydrological analyses were based on long-term monitoring data from Slovenian Environmental Agency and partly on the authors' own database of field measurements and knowledge of the areas concerned. Consequently, the conceptual framework and the key criterion for the determination and recording of areas subject to temporary flooding were developed. A systematic survey reveals the significance of the extent, duration and frequency of flooding. Evaluation results are useful to designers of various water policies and management mechanisms for flood mitigation and protection of special habitats. Observing trends in the spatial and temporal dynamics of flood levels is also valuable for understanding how and to what extent karst aquifers are vulnerable to environmental changes. Finally, the analyses also enable prediction of the effects of these changes on other parts of the environment (e.g., hydrophilic habitats).</p>

Tree species from Andean forest are complementary in their effect on ecohydrological processes

<p>Vegetation affects water balance partitioning via effects on incoming precipitation, local radiation balance and hydrological dynamics of soil. The extent of these effects is determined by plant functional traits. Commonly, the role of plant species on hydrological regulation has been assessed considering vegetation as homogeneous cover, even more, that approach underestimates the importance of species in this process. Nevertheless, in recent years, new focus has been placed on species study based on their functional traits and their roles in ecosystem functions as hydrological regulation. Still new tendencies are considering vegetation cover consisting of different species, each of them having different effects on hydrological regulation because they have different functional traits. In an 8-year old ecosystem restoration project established in Medellín (Colombia), we explored the relations between plant functional traits of 10 dominant species and ecohydrological processes that determine precipitation partitioning in the canopy via stemflow and throughfall. Here we show that functional traits describing tree crowns are significantly related with stemflow and throughfall. Our species exhibit differences in their functional traits and ecohydrological processes, forming a gradient of variation of ecohydrological processes and crown functional traits: from wide and less dense crowns in <em>Alnus acuminata</em> to smaller but more dense crowns in <em>Quercus humboldtii</em>, related with less throughfall temporal variability, and less stemflow temporal variability, respectively; the other species are placed along this gradient. This result suggests a complementary effect of species on the hydrological processes and consequently on the hydrological function, highlighting the importance of considering species diversity on hydrological regulation assessment. More specifically, our results emphasize the need to include information about the effects of species planted in ecological restoration projects over ecohydrological processes, via ecological criteria such as plant functional traits. This approach permits a more objective and complete study of hydrological regulation that brings key information for an adequate ecosystem management and restoration based on ecological roles of species that, through biological diversity, optimize ecosystem functions and services.</p>

Study on Relationship of Land Cover Changes and Ecohydrological Processes of the Tuul River Basin

The Tuul River Basin is the most important socioeconomic and political base area of Mongolia. Therefore, studying the interrelationships between changes in the ecohydrological processes of this basin and its land cover is of great importance for maintaining sustainability and the environment. This study investigated the annual average air temperature, total annual precipitation, and river discharge variability, and land cover changes at selected stations of the basin by using the hydrometeorological analysis, satellite analysis, and land cover determination statistical analysis. During the study period, the average annual air temperature rose from −1.5 °C to +0.3 °C (1.8 °C 361 °C). The average annual precipitation exhibits relatively low change during this period. River discharge varied during the study period. A significant decreasing trend in river discharge was observed at the Terelj (φ = −2.72) and Ulaanbaatar (φ = −5.63) stations, whereas the other stations, Altanbulag, Lun, and Orkhontuul, showed a significant increasing trend. During the study period, changes in land cover were directly related to main hydrometeorological parameters. Between 2000 and 2020, the amount of grassland decreased by 319.67 km2, while the area of water bodies increased by 28.36 km2. In the study area, mainly water bodies and sensitive areas of the land cover types were changed due to changes in precipitation. Studies in the arid and semiarid regions of Central Asia show that changes of ecohydrological processes have a significant impact on land cover changes.

Integrating Landscape Metrics and Hydrologic Modeling to Assess the Impact of Natural Disturbances on Ecohydrological Processes in the Chenyulan Watershed, Taiwan

The Chenyulan watershed, located in the central mountain area of Taiwan, has been suffering from earthquakes, typhoons, and heavy rainfalls in recent decades. These sequential natural disturbances have a cumulative impact on the watershed, leading to more fragile and fragmented land cover and loss of capacity of soil water conservation. In this study, the Soil and Water Assessment Tool (SWAT) and a landscape metrics tool (FRAGSTATS) were used to assess the direct impact (e.g., by annual rainfall) and indirect impact (e.g., by landscape configuration and composition) of natural disturbances on the ecohydrological processes of the Chenyulan watershed. Six SPOT satellite images from 2008 to 2013 were analyzed by using the nearest feature line embedding (NFLE) approach and reclassified into six land cover types: forest, cultivated land, grassland, river, landslide, and built-up. Forest was found to have the largest patch size, indicating that it is more resilient to disturbances, while agricultural land tended to expand from the river side toward the hill. Two land cover change scenarios were compared in the SWAT model. The results showed that there was no significant difference in simulated streamflow during 2004–2015 and sediment loading during 2004–2009; however, the model performed better for sediment loading during 2010–2015 with dynamic land cover change (coefficient of determination (R2) = 0.66, Nash-Sutcliffe efficiency coefficient (NSE) = 0.62, percent bias (PBIAS) = 10.5%, root mean square error observation standard deviation ratio (RSR) = 0.62) than with constant land cover (R2 = 0.61, NSE = 0.54, PBIAS = −17.3%, RSR = 0.68), indicating that long-term land cover change should be considered in hydrologic modeling. Changes in landslides during 2008–2013 were found to significantly affect ecohydrological processes, especially after 2011. In general, annual precipitation plays a dominant role, and landscape composition had by far the strongest influence on water yield and sediment yield compared to landscape configuration. The results can be useful for understanding the effects of land cover change on ecohydrological processes in the Chenyulan watershed and the potential impact of ecohydrological changes on the environment and public health.