The evaporation from a lake is to be calculated by the water balance method. Inflow to the lake occurs through three small rivers, A, B and C. The outflow occurs through the river D. Calculate the evaporation from the lake surface during summer (May – August) if the water level was +571.04 m
on May 1 and +571.10 m
on August 31. The lake surface area is 100 km$^2$. The precipitation P during the period was 100 mm. Average inflows and outflow are given below:
River Catchment(km2) Average Q(m3/s)
A 150 15
B 120 20
C 130 17
D 45
This is the way I have attempted it:
Total Inflow ( by A, B, C) $= 52\ \mathrm{m^3/s}$.
Outflow (by D, assuming that D is fed by the lake) $= 45\ \mathrm{m^3/s}$.
Inflow - Outflow = Change in storage (which is in turn change in elevation × area)
Net Inflow into the lake = Inflow by 3 rivers + Precipitation $$ = (52\ \mathrm{m^{3}/s} \times 122 \times (30\ \mathrm{days}+31+30+31)\times24\times60\times60 + 0.1\ \mathrm{m}\times100\times10^3 \mathrm{m}^2) $$
Total Outflow = Outflow + Evaporation $$ = (45\times122\times24\times60\times60 + E), $$ where E is the component to be found.
Change in storage = Change in elevation × Area $$ = (571.10 - 571.04)\times100 \times 10^6 E = 0.68\ \mathrm{m} $$