Water Balance - Soil Moisture Surplus

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).

Table 10.4 Soil Moisture Surplus - Rockford, IL
Field Capacity = 90 mm
	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

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).

Self Assessment

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?

Choose one of the answers below by clicking a button

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