Figure 22: Annual Evaporation (mm/yr)
Soil water balancing refers to the partitioning of precipitation into
evaporation, and runoff or recharge by using a water balance calculation
applied to a volume of soil. Seann
M. Reed has developped a series of Avenue
scripts and exercies to determine the soil water balance.
The input data needed for the soil water balance were the temperature,
radiation, water holding capacity and precipitation grids obtained from
the Digital Chart of the World, and the extent of the watershed. The Avenue
scripts use the Priestley-Taylor equations to calculate the potential evaporation
and then the soil moisture budget. However, it should be noted that the
script used here do not take snow into consideration. Therefore, when the
potential evaporation was calculated some cells contained negative values.
These negative values were changed to zero in order to run the soil moisture
budget.
The final evaporation values for the area can be seen below. As it can
be expected, the evaporation upstream in mountaineous areas is low, while
evaporation in the low plains is high.
Figure 22: Annual Evaporation (mm/yr)
The first calculation of water surplus was calculuated using Seann Reed's
program. This program did not include snowmelt. The figure below show the
results of that program. Surplus is defined as water which does not evaporate
or remain in soil storage and includes both surface and subsurface runoff.
surplus = precipitation - evaporation - (change in storage)
Figure 23: Annual Water Surplus Without Snow Melt (mm/yr)
Because of the high altitudes in Central Asia, it was important to incorporate snow melt into the Fortran program. Snow Melt was defined as follows:
If temp >= 2 then rain
Else snow
Melt = 2.63 + 2.55 (temp) + 0.0912(temp)(rain)
The snow storage at any time is equal to the snow storage at time before plus the fallen snow within that time minus the snowmelt.
sstori+1 = sstori + snow - melt
The next step was to make sure that the snow storage is never below zero.
If sstori+1 <0 then sstori+1 = 0
So that all the snow melt did not become runoff, a final equation was added:
Runoff = melt * water holding capacity
The results of this program are shown below:
Figure 24: Annual Water Surplus With Snow Melt (mm/yr)
As can be seen the water surplus is higher using the snow melt program