The quantification of accessible water storage changes takes place using the water mass balance approach. The terrestrial water balance that the water equation for GRACE looks to determine is made up of components that can be isolated individually. The composition of components, therefore, includes soil moisture, surface water, snow water equivalent and ground water. This water balance (total surface water) is calculated by adding soil moisture, surface water, snow water equivalent and ground water. However, there is usually a deviation from the calculated values due to anomalies. An anomaly in the context of the study refers to the deviations observed on the total water storage for a given period of study. To calculate an anomaly, one needs to make observations on the deviations evident from the mean of the total water storage. Thus, the formula is as follows,
Where: Change in accessible water= Total water storage- Change in snow water equivalent- change in soil moisture
The author believes that the GRACE equation offers an engineer to estimate with accuracy the amount of water that can be lost from a dam or basin. As the results of the studies conducted affirm, the comparisons from the GRACE-based estimates and groundwater depletion rates reported in the summaries show many similarities. The formula, therefore, becomes more reliable to be used by engineers in the analysis of water management. The analysis of the GRACE equation can inform the use of water in areas affected by calamities particularly drought-stricken regions. For example, engineers in charge of dams or basins used to generate hydroelectric power can determine with certainty the active surface water and surplus inflows within the reservoirs. The information can be used in ensuring a constant supply of electricity for consumers while also avoiding instances where the reservoir goes dry or experiences overflows.
Castle, S. L., Thomas, B. F., Reager, J. T., Rodell, M., Swenson, S. C., & Famiglietti, J. S. (2014). Groundwater depletion during drought threatens future water security of the Colorado River Basin. Geophysical Research Letters, 41(16), 5904-5911.