Arast M, Ranjbar A, Mousavi SH, Abdollahi K and Honarbakhsh A

Abstract

Data from the Gravity Recovery and Climate Experiment (Grace) satellite and stations were used to estimate variations
of the monthly groundwater level in the Zayanderud basin, Iran, over the period 2000–2018. In addition, the annual
and seasonal storage of groundwater in this basin was estimated and verified using Grace satellite data, and
compared with data from the previous rainfall. The results showed that the current groundwater level depends on
the amount of rainfall in past years. When rainfall is on a downtrend, the trend of underground water storage
fluctuations is affected by rainfall to a greater extent. In addition to the time trend, the pattern of the normalised
difference vegetation index (NDVI) depends on the amount of groundwater storage variations in the Zayanderud
basin. The lowest NDVI (0·078) in 2016 coincided with the lowest amount of underground water storage and recharge
rate (27·36 cm). Statistical correlation analysis of Grace satellite data and recorded data from wells indicated a root
mean square error of 2·23 cm, a mean absolute error of 3·28 cm and a mean bias error of 1·02 cm on the seasonal
scale. The results show that Grace data can provide a good understanding of long-term variations when studying the
relationship between groundwater level and rainfall, especially in large basins.