Assessment of Physical Water Scarcity in Africa Using GRACE and TRMM Satellite Data

Abstract

The critical role of water in enabling or constraining human well-being and socioeconomic activities has led to an interest in quantitatively establishing the status of water (in)sufficiency over space and time. Falkenmark introduced the first widely accepted measure of water status, the Water Scarcity Index (WSI), which expressed the status of the availability of water resources in terms of vulnerability, stress, and scarcity. Since then, numerous indicators have been introduced, but nearly all adopt the same basic formulation; water status is a function of “available water” resource—by the demand or use. However, the accurate assessment of “available water” is difficult, especially in data-scarce regions, such as Africa. In this paper, therefore, we introduce a satellite-based Potential Available Water Storage indicator, PAWS. The method integrates GRACE (Gravity Recovery and Climate Experiment) satellite Total Water Storage (TWS) measurements with the Tropical Rainfall Measuring Mission (TRMM) precipitation estimates between 2002 and 2016. First, we derived the countries’ Internal Water Storage (IWS) using GRACE and TRMM precipitation data. Then, the IWS was divided by the population density to derive the PAWS per capita. Following the Falkenmark thresholds, 54% of countries are classified in the same water vulnerability status as the AQUASTAT Internal Renewable Water Resources (IRWR) method. Of the remaining countries, PAWS index leads to one or two categories shift (left or right) of water status. The PAWS index shows that 14% (~160 million people) of Africa’s population currently live under water scarcity status. With respect to future projections, PAWS index suggests that a 10% decrease in future water resources would affect ~37% of Africa’s 2025 population (~600 million people), and 57% for 2050 projections (~1.4-billion people). The proposed approach largely overcomes the constraints related to the data needed to rapidly and robustly estimate available water resources by incorporating all stocks of water within the country, as well as underscores the recent water storage dynamics. However, the estimates obtained concern potential available water resources, which may not be utilizable for practical, economic, and technological issues.

Satellite Remote Sensing of Precipitation and the Terrestrial Water Cycle in a Changing Climate

“…Keen attention must be paid to areas that are more affected by water scarcity, such as the African continent [324,325], where water shortages may concern 37% of the 2025 population (~600 million people) and 57% in 2050 (~1.4 billion people), under the hypothesis of a decrease in water resources by 10%. Concerning water management, Biswas and Tortajada [326] point out unambiguously that "countries will have to manage their water significantly better, because they will have no other choice".…”

Trend Dynamics of GRACE Terrestrial Water Storage in the Nile River Basin

“…Then a fixed sample grid to GRACE CSR-M product was used to sample the corresponding trend value at a fixed spatial resolution of 1.0-degree grid. The associated uncertainty in each variable was expressed using standard deviations of the residual component following standardized approaches [7,39,69,79,80]. Table 1.…”

“…TWSA, and several independent variables can be analyzed based on either the raw values of the process or on the dominant component characterizing the time series, such as trends, cycles, and seasonality [59,[81][82][83][84][85]. With specific respect to TWSA across Africa, previous research (e.g., [7,16,18,19,24,25,35,38,86]) showed that large parts of the continent are experiencing statistically significant declining TWSA trends. The studies further established that the trend component provides a robust means of investigating the relationship between TWSA and the driving hydrologic variables.…”

“…Gravitational field anomalies measured by GRACE (Gravity Recovery and Climate Experiment) satellites have yielded estimates of changes in the terrestrial water storage (TWS) across various temporal and spatial scales [1][2][3][4][5][6][7][8][9]. The TWS estimates integrate the effects of all processes that impact water storage and flux at the Earth's surface down to the deepest groundwater aquifer [2,[10][11][12][13].…”

“…Complex interactions among these processes determine the amount, timing, and variations in TWSA. Consequently, GRACE-derived TWS anomalies have been used in an expanding array of studies to investigate water scarcity [7], groundwater depletion [2,[10][11][12][13], lake storage variations [19,21,24,25], snow and glacial mass changes [26,27], as well as floods [28,29] and droughts [15,[30][31][32][33][34]. These processes vary significantly over time and space, and the degree of variability increases as the study domain increases.…”