An improved geography of surface water abundance in lakes and reservoirs

Abstract

The reserves of fresh water on the earth are of critical environmental and social importance. The great mass of liquid freshwater, which is readily accessible to human beings, resides in major surface stocks such as lakes, ponds, and reservoirs. Unfortunately, because of the extensive distribution and dynamic nature of global surface water bodies, our knowledge of the accurate extent, quantity, and quality of global lake systems remains surprisingly poor. River flow and storage in lakes/reservoirs are two main surface water resources for human societies. When river flow experiences high stress due to increasing human water demand, water storage in lakes and reservoirs can provide fresh water to mitigate the stress of river flow. Therefore, how river flow and lake water storage are distributed across the continental surface and what is the balance between them are crucially important. The overarching objective of this thesis is to improve the understanding of the physical abundance of surface water in lakes and reservoirs. This goal was achieved by answering four specific questions: 1. Distribution: Where are freshwater lakes, and where are saline lakes? 2. Abundance: How many freshwater lakes and saline lakes populate across the Earth’s surface, and how much area do they cover? 3. Storage: What are the total water storages in freshwater and saline lakes? 4. Storage versus flows: What is the spatial contrast between surface river flow and lake storage? The question related to the distribution and the abundance of global freshwater and saline lakes was answered by integrating hydrological, spectral, climatological, and literature evidence on various geographic settings. I systematically classified global lakes and reservoirs into freshwater and saline categories from a high-resolution circa-2000 lake dataset that documents perennial water bodies greater than 0.4 hectares (Sheng et al., 2015; Sheng et al., 2016). In order to understand the storage of global lakes/reservoirs, I then estimated the mean depth, volume, and associated uncertainties for each lake/reservoir using geo-statistics, GIS-based terrain analysis, and existing literature. To understand the change of the distribution and the abundance of global lakes during recent 15 years, attributes of each inventoried lake were updated on the recently-produced circa-2015 dataset (Sheng et al., 2015; Sheng et al., 2016) based on the results of freshwater/saline classification and storage estimation for circa 2000. To explore the spatial contrast between lake storage and river flow, which are two major surface water recourses for human society, the lake storage of circa-2000 and the average annual discharge during 1990–2010 simulated by the PCRaster Global Water Balance (PCR-GLOBWB) hydrological model were compared on the spatial scales of both individual drainage basins and 0.1° geographic grid. This research improves our understanding of water abundance in global lakes and reservoirs by providing their freshwater/saline classification, number and storage distributions, and the comparison of their storages with river flow. Specifically, results for the first three questions reveal two unprecedentedly detailed inventories of global lake extents and storage with freshwater and saline classification. Our classified global lake inventories conclude 8–9 million lakes, with a total area of 3,002 thousand km² and a total volume of 189 (±6) thousand gigatons (Gt) in the circa-2000 inventory, and a total area of 2,976 thousand km² and a total volume of 190 (±6) thousand Gt in the circa-2015 inventory. Among them, saline lakes account for less than 2% in number, about a quarter in area and half in volume. The result for the fourth question shows the spatial disparity among surface water resources. Two-scale datasets that contain freshwater lake storage, saline lake storage, and average annual river discharge (1990–2010) were generated to improve our understanding of the spatial distribution among the three factors in different regions. These spatially-explicit contrasts show that high freshwater storage tends to coexist with high flow, except high-latitude (pan-Arctic) regions where lake distributions are extensive and thus water storage largely exceeds the flow. Saline water storage is spatially less or negatively correlated to freshwater storage or discharge, thus implicating arid/semiarid climates and water-stressed regions. In general, river flow and freshwater lakes are the main water sources in humid regions, but saline lakes are dominant in arid/semi-arid regions. The detailed spatial cross-tabulations between surface river flow and storage provide an important guidance for potential water resource management.