Information transfer for hydrologic prediction in ungauged river basins.

摘要

Long-term measurements of river streamflow are essential for numerous applications in water resources. However, in many parts of the world, developed as well as developing, rivers are not gauged for continuous monitoring. Streamflow prediction at such "ungauged" river catchments requires information transfer from gauged catchments that are perceived to be hydrologically similar to them. Achieving good predictability at ungauged catchments requires an in-depth understanding of the physical and climatic controls on hydrologic similarity among catchments. This dissertation attempts to gain a better understanding of these controls through three independent research studies that use data from catchments across the continental United States.;In the first study, I explore whether streamflow similarity among nearby catchments is preserved across flow conditions. Catchments located across four river basins in the northeast United States are analyzed to quantify the spatio-temporal variability in streamflows across flow percentiles. Results show that similarity in catchment stream response is dynamic and highly dependent on flow conditions. Specifically, within each of the four basins, the coefficient of variation is high at low flow percentiles and gradually reduces for higher flow percentiles. Greater similarity in streamflows is observed during the winter and spring (wet) seasons compared to the summer and fall (dry) seasons. This study concludes that high variability at low flows is controlled by the dominance of high evaporative demand, whereas low variability at high flows is controlled by the dominance of precipitation input relative to evapotranspiration.;In the second study, I examine whether streamflow similarity among catchments exists across a wide range of climatic and geographic regions. Data from 756 catchments across the United States is used and daily streamflow at each catchment is simulated using distance-based streamflow interpolation from neighboring catchments. With this approach, high predictability at a catchment indicates that catchments in its vicinity have similar streamflows. Results show that high predictability catchments are mainly confined to the Appalachian Mountains, the Rocky Mountains, and Cascade Mountains in the Pacific Northwest. Low predictability catchments are located mostly in the drier regions of US to the west of Mississippi river. Results suggest that high streamflow similarity among nearby catchments (and therefore, good predictability at ungauged catchments) is more likely in humid runoff-dominated regions than in dry evapotranspiration-dominated regions. I further find that having higher density and/or closer distance of gauged catchments near an ungauged catchment does not necessarily guarantee good predictability at an ungauged catchment.;In the third study, my goal is to identify what constitutes the essential information that must be transferred from gauged to ungauged catchments in order to achieve good model predictability. A simple daily time-step rainfall-runoff model is developed and implemented over 756 catchments located across the United States. For characterization of hydrologic similarity between the gauged and ungauged catchments, the methods based on physical proximity and spatial proximity measures are compared. Results show that the rainfall-runoff model simulates well at catchments in humid low-energy environments, most of which are located in the eastern part of the US, the Rocky Mountains, and along the west coast (to the west of Cascade Mountains). Within these regions, transfer of the parameter characterizing hydrograph recession provides reliable streamflow predictions at ungauged catchments, with a loss in prediction efficiency of less than 10% in most catchments. Results further show that transferring model parameters from gauged catchments based on spatial proximity measures provides better streamflow predictability at ungauged catchments than that based on physical proximity measures.;The results presented in this dissertation show that climate exerts a strong control on hydrologic similarity among catchments. This has ramifications for determining the regions in which hydrologic information can be reliably transferred from gauged catchments to make predictions at ungauged catchments. The results further suggest that an understanding of the interaction between climate and topography is essential for quantifying the spatial variability in catchment hydrologic behavior at a regional scale.