Abstract:
This study integrates time-variable Gravity Recovery and Climate Experiment
(GRACE) gravimetric measurements and Global Land Data Assimilation System
(GLDAS) land surface models (LSM) in order to understand the inter-annual
variations and groundwater storage changes (GWSC) in the Nzoia River Basin
in Kenya, using the water balance equation and parameters. From averaged
GRACE and GWSC data, the results showed that over the 10-year period, the
basin experienced a groundwater depth gain of 6.38 mm year −1 , which is equiva-
lent to aquifer recharge of 298 million cubic metres (mcm) year −1 . The deseaso-
nalized groundwater variation analysis gave a net gain in groundwater storage of
6.21 mm year −1 that is equal to a groundwater recharge gain of 290 mcm year −1 .
The observed results are comparable to the groundwater safe yield of 330 mcm
year −1 as estimated by the Water Resource Management Authority in Kenya.
Through cross-plotting and analysis with averaged satellite altimetry data and in
situ measurements from rainfall and streamflow discharge, the total water storage
change (TWSC) and GWSC in the basin were consistent and closely correlated in
variation trends. The inter-annual standard deviation of groundwater change was
determined as ±0.24 mm year −1 , which is equivalent to 85% degree of confidence
in the obtained results. The results in this study show that GRACE gravity-
variable solutions and GLDAS-LSM provide reliable data sets suitable for the
study of small to large basin groundwater storage variations, especially in areas
with scarce and sparsely available in situ data.
