Soil-water budgets
 Considers
all forms of soil moisture for a
given region
 Indicates
for plants
how much moisture is available
Components of soil-water budget
 Precipitation
input (PRECIP)
 Actual evapotranspiration (ACTET)
 Potential evapotranspiration (POTET)
 Deficit (DEFIC)
 Surplus (SURPL)
 Soil-moisture storage (ΔSTRGE)
Evapotranspiration
 Evaporation

The change of liquid water into water vapor
 Transpiration

A cooling mechanism in plants
 Evapotranspiration
is therefore the release
of water vapor from plants in order to cool
them
 Evapotranspiration is highest on hot and
sunny days
Potential Evapotranspiration
 The
amount of water that would be
evapotranspired under specific
temperature and moisture conditions
 This
measurement ignores whether water
is available for transpiration or not
 It
is the DEMAND!
INPUTS
OUTPUTS
Deficit
 Will
occur if the requirements for
potential evapotranspiration are not
met
Surplus
 Will
occur if the requirements for
potential evapotranspiration are met
and there is extra moisture available
Soil moisture storage
 Amount

of moisture available to plants
Change storage = amount of water taken
from ST to help meet PE when P is lacking
Water balance technique
 Precipitation
(P) vs potential
evapotranspiration (PE)
P
= AE + S + ST
 PE = AE + D
 AE = amount of water actually used by
plants so if water is in good supply AE =
PE…. IF WATER IS LACKING THEN AE =
P + ST
POTET
Determining
water budgets
 For
each month
the following can
be plotted



PRECIP
POTET
ACTET
Water Balance
 Plants
 Once
ST
will not use more than PE
PE is used up plants will take it from
1952 Mississippi
1952 IS IT A WET OR A DRY YEAR?
J
PE
F
M
A
M
J
JL
A
S
O
N
D
1
1
1.2
2.3
4.6
7.1
7.3
6.2
4.3
1.5
1
0.5
P
3.5
3.1
5.5
2.3
3.2
0.3
2.1
2.7
1.2
0.3
2.7
4.6
P-PE
2.5
2.1
4.3
0
-1.4
-6.8
-5.2
-3.5 -3.1 -1.2
1.7
4.1
ST
6
6
6
6
4.6
0
0
0
0
0
1.7
5.8
∆ST
0
0
0
0
-1.4
-4.6
0
0
0
0
1.7
4.1
AE
1
1
1.2
2.3
4.6
4.9
2.1
2.7
1.2
0.3
1
0.5
D
0
0
0
0
0
2.2
5.2
3.5
3.1
1.2
0
0
S
2.5
2.1
4.3
0
0
0
0
0
0
0
0
0