Integration of altimetric lake levels and GRACE gravimetry over Africa: Inferences for terrestrial water storage change 2003–2011

Abstract

Terrestrial water storage (TWS) change for 2003-2011 is estimated over Africa from GRACE gravimetric data. The signatures from change in water of the major lakes are removed by utilizing kernel functions with lake heights recovered from retracked ENVISAT satellite altimetry. In addition, the contribution of gravimetric change due to soil moisture and biomass is removed from the total GRACE signal by utilizing the GLDAS land surface model. The residual TWS time series, namely groundwater and the surface waters in rivers, wetlands, and small lakes, are investigated for trends and the seasonal cycle using linear regression. Typically, such analyses assume that the data are temporally uncorrelated but this has been shown to lead to erroneous inferences in related studies concerning the linear rate and acceleration. In this study, we utilize autocorrelation and investigate the appropriate stochastic model. The results show the proper distribution of TWS change and identify the spatial distribution of significant rates and accelerations. The effect of surface water in the major lakes is shown to contribute significantly to the trend and seasonal variation in TWS in the lake basin. Lake Volta, a managed reservoir in Ghana, is seen to have a contribution to the linear trend that is a factor of three greater than that of Lake Victoria despite having a surface area one-eighth of that of Lake Victoria. Analysis also shows the confidence levels of the deterministic trend and acceleration identifying areas where the signatures are most likely due to a physical deterministic cause and not simply stochastic variations.

Large-Scale Total Water Storage and Water Flux Changes over the Arid and Semiarid Parts of the Middle East from GRACE and Reanalysis Products

“…Estimations of X Surface water can be introduced by converting altimetry measurements to storage following, e.g., Moore and Williams (2014). Groundwater (X Groundwater ) estimation is missing in land surface models such as GLDAS and therefore is estimated here after solving the inversion.…”

“…3.4) over a box 29°-63°E and 24°-43°N (that includes the seven basins), covering the period 2003-2013. Information on the surface water compartment is obtained by converting altimetry measurements over the Caspian Sea, Persian and Oman Gulfs, Lake Urmia (37 42 0 N 45 19 0 E), and Lake Tharthar (33 58 0 N 43 11 0 E) to storage changes following Moore and Williams (2014). All altimetry measurements are downloaded from (http://www.pecad.fas.…”

Long-Term Water Storage Changes of Lake Volta from GRACE and Satellite Altimetry and Connections with Regional Climate

“…GRACE estimates have been used to study long-term TWS changes at both global and regional scales [9][10][11][12][13][14]. For example, Humphrey et al [12] investigated the relative importance of linear trend and nonlinear interannual variability in long-term TWS changes throughout the world.…”

“…Awange et al [9] found out that a catchment area of 4 × 4 • including Lake Naivasha in East Africa lost water at a rate of 1.4 cm/year for the period of 2002-2010. Ahmed et al [11] and Moore and Williams [13] found that large sectors of Africa are undergoing significant TWS variations (+44 mm/year to −15 mm/year) due to natural and anthropogenic causes.…”

“…Ahmed et al [11] and Moore and Williams [13] have attributed this to the impoundment of water by Akosombo Dam. Ndehedehe et al [16] examined space-time characteristics of droughts over the VRB using GRACE data and multiple drought indices.…”