Thirdly, a crude approximation of the total amount of freshwater
that is available in the region directly from precipitation is made by
multiplying the average annual precipitation across the region by
the surface area of the water supply hinterland. In the fourth step,
we need to adjust this total precipitation by average losses due to
evapotranspiration across the supply area and some factor representing
the “acceptable” but complex waterway environmental
flows required for ecosystem services and integrity. Given the
profound potential costs of exceeding environmental flows, and
inaccuracies in evapotranspiration levels, we utilize maximal estimates
for these withdrawals.