10.3.1: Computing a Soil - Moisture Budget
The best way to understand how the water balance works is to actually calculate a soil water budget. We'll use Rockford, Illinois which is located in the humid continental climate of northern Illinois. Rockford lies on the northern edge of the prairie and mixes with deciduous forest. This vegetation has been nearly completely replaced with agriculture. A knowledge of soil moisture status is important to the agricultural economy of this region that produces mostly corn and soy beans.
To work through the budget, we'll take each month (column) one at a time. It's important to work column by column as we're assessing the moisture status in a given month and one month's value may be determined by what happened in the previous month.
|
J |
F |
M |
A |
M |
J |
J |
A |
S |
O |
N |
D |
Year |
|
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
P |
50 |
49 |
66 |
78 |
100 |
106 |
88 |
84 |
86 |
73 |
56 |
45 |
881 |
|
PE |
0 |
0 |
5 |
40 |
84 |
123 |
145 |
126 |
85 |
44 |
8 |
0 |
531 |
|
P-PE |
50 |
49 |
61 |
38 |
16 |
-17 |
-57 |
-42 |
1 |
29 |
48 |
45 |
|
|
ΔST |
0 |
0 |
0 |
0 |
0 |
17 |
57 |
16 |
1 |
29 |
48 |
12 |
|
|
ST |
90 |
90 |
90 |
90 |
90 |
73 |
16 |
0 |
1 |
30 |
78 |
90 |
|
|
AE |
0 |
0 |
5 |
40 |
84 |
123 |
145 |
100 |
85 |
44 |
8 |
0 |
634 |
|
D |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
26 |
0 |
0 |
0 |
0 |
26 |
|
S |
50 |
49 |
61 |
38 |
16 |
0 |
0 |
0 |
0 |
0 |
0 |
33 |
258 |
Soil Moisture Recharge
We'll start the budget process at the end of the dry season when precipitation begins to replenish the soil moisture, called soil moisture recharge , in September. At the beginning of the month the soil is considered dry as the storage in August is equal to zero. During September, 86 mm of water falls on the surface as precipitation. Potential evapotranspiration requires 85 mm. Precipitation therefore satisfies the need for water with one millimeter of water left over (P-PE=1). The excess one millimeter of water is put into storage (ΔST=1) bringing the amount in storage to one millimeter (August ST =0 so 0 plus the one millimeter in September equals one millimeter). Actual evapotranspiration is equal to potential evapotranspiration as September is a wet month (P>PE). There is no deficit during this month as the soil now has some water in it and no surplus as it has not reached its water holding capacity.
During the month of October, precipitation far exceeds potential evapotranspiration (P-PE=29). All of the excess water is added to the existing soil moisture (ST (September) + 29 mm = 30 mm). Being a wet month, AE is again equal to PE.
Calculating the budget for November is very similar to that of September and October. The difference between P and PE is all allocated to storage (ST now equal to 78 mm) and AE is equal to PE.
Soil Moisture Surplus
During December, Rockford is deep in the grip of winter. Potential evapotranspiration has dropped to zero as plants have gone into a dormant period thus reducing their need for water and cold temperatures inhibit evaporation. Notice that P-PE is equal to 45 but not all is placed into storage. Why? At the end of November the soil is within 12 mm of being at its field capacity. Therefore, only 12 millimeters of the 45 available is put in the soil and the remainder runs off as surplus (S=33).
Given that the soil has reached its field capacity in December, any excess water that falls on the surface in January will likely generate surplus runoff. According to the water budget table this is indeed true. Note that P-PE is 50 mm and ΔST is 0 mm. What this indicates is that we cannot change the amount in storage as the soil is at its capacity to hold water. As a result the amount is storage (ST) remains at 90 mm. Being a wet month (P>PE) actual evapotranspiration is equal to potential evapotranspiration. Note that all excess water (P-PE) shows up as surplus (S=50 mm).
Similar conditions occur for the months of February, March, April, and May. These are all wet months and the soil remains at its field capacity so all excess water becomes surplus. Note too that the values of PE are increasing through these months. This indicates that plants are springing to life and transpiring water. Evaporation is also increasing as insolation and air temperatures are increasing. Notice how the difference between precipitation and potential evapotranspiration decreases through these months. As the demand on water increases, precipitation is having a harder time satisfying it. As a result, there is a smaller amount of surplus water for the month.
Surplus runoff can increase stream discharge to the point where flooding occurs. The flood duration period lasts from December to May (6 months), with the most intense flooding is likely to occur in March when surplus is the highest (61 mm).
If the field capacity of this soil was larger, will the monthly surplus be larger or smaller than it is under its present field capacity?
- Answer
-
Smaller - If the field capacity is larger, the soil will take longer to become saturated and retain more water resulting in less surplus runoff.
Soil Moisture Utilization
By the time June rolls around, temperatures have increased to the point where evaporation is proceeding quite rapidly and plants are requiring more water to keep them healthy. As potential evapotranspiration is approaching its maximum value during these warmer months, precipitation is falling off. During June P-PE is -17 mm. What this means is precipitation no longer is able to meet the demands of potential evapotranspiration. In order to meet their needs, plants must extract water that is stored in the soil from the previous months. This is shown in the table by a value of 17 in the cell for ΔST (change in soil storage). Once the 17 m is taken out of storage (ST) it reduces its value to 73.
The month of June is considered a dry month (P<PE) so AE is equal to precipitation plus the absolute value of ΔST (P + |ΔST|). When we complete this calculation (106 mm + 17 mm = 123 mm) we see that AE is equal to PE. What this means is precipitation and what was extracted from storage was able to meet the needs demanded by potential evapotranspiration. Note that there is no surplus in June as the soil moisture storage has dropped below its field capacity. There is still no deficit as water remains in storage. The calculations for July is similar to June, just different values. Note that by the time July ends, water held in storage is down to a mere 16 mm.
Soil Moisture Deficit
August, like June and July, is a dry month. Potential evapotranspiration still exceeds precipitation and the difference is a -42 mm. Up until this month there has been enough water from precipitation and what is in storage to meet the demands of potential evapotranspiration. However, August begins with only 16 mm of water in storage (ST of July). Thus we'll only be able to extract 16 mm of the 42 mm of water needed to meet the demands of potential evapotranspiration So, of the 42 mm of water we would need (P-PE) to extract from the soil. In so doing, the amount in storage (ST) falls to zero and the soil is dried out. What happens to the remaining 26 mm of the original P-PE of 42? The unmet need for water shows up as soil moisture deficit. In other words, we have not been able to meet our need for water from both precipitation and what we can extract from storage. AE is therefore equal to 100 mm (84 mm of precipitation plus 16 mm of ΔST).
So what is a farmer to do if their crops cannot obtain needed water from precipitation or soil moisture storage....they irrigate. Irrigation water usually is pumped from groundwater supplies held in aquifers deep below the surface or from nearby streams (if stream flow is sufficient to provide needed water). The amount of irrigation water required is the amount of the soil moisture deficit.