3.2.3. Storage Estimation

Reservoir storage can be estimated based on the remotely sensed water surface area and elevation using Equation (1):

$$V\_{RS} = V\_{\mathbb{C}} - \left(A\_{\mathbb{C}} + A\_{RS}\right)\left(H\_{\mathbb{C}} - H\_{RS}\right)/2\tag{1}$$

where *VC*, *AC*, and *HC* represent the storage, area, and water elevation at capacity, respectively. *VRS*, *ARS*, and *HRS* are the remotely sensed storage, area, and water height at the monitoring time.

In this MODIS-SRTM algorithm, since *H*RS can be calculated by applying the *A*-*H* relationship to the MODIS area estimation (i.e., *ARS*), the reservoir storage is calculated through Equation (2) (which was transformed from Equation (1)).

$$V\_{R\mathcal{S}} = V\_{\mathcal{C}} - \left(A\boldsymbol{\varsigma} + A\boldsymbol{\kappa}\boldsymbol{\varsigma}\right)\left(A\boldsymbol{\varsigma} - A\boldsymbol{\kappa}\boldsymbol{\varsigma}\right)\boldsymbol{k}/2\tag{2}$$

Using the methods explained in this section, the reservoir storage was calculated for the 28 selected reservoirs in South Asia from 2000 to 2015. Using the Hirakud reservoir as an example, Figure 4b compares the time series of reservoir storage from this MODIS-SRTM algorithm with that from the MODIS-ICESat algorithm by Zhang et al. [25]. Results sugges<sup>t</sup> that these two sets of storage estimations are in good agreement. However, compared with the MODIS-ICESat-based algorithm, the storage values from this study tend to be underestimated (due to the different *A-H* relationships). To better understand the error statistics of these two approaches, validations using gauge data were conducted and are reported on in Section 4.1.
