WATER RESOURCES OF ORANGE COUNTY 33
VARIATION
Part of the difficulty in managing the water resources of an area stems from variations in the amount of water stored on and beneath the earth's surface in the area. These variations are brought about because rainfall is extremely variable and intermittent, while evaporation, transpiration, surface outflow, and underground outflow though also variable are relatively continuous. Figure 9 shows the average discharge in cfs (cubic feet per second) per square mile for each year of record at three stations draining parts of Orange County. Figure 10 shows the annual rainfall at Orlando for this period. Comparison of these two figures reveals that the pattern of variations in runoff and rainfall are similar but not identical. The years of high flow agree well with the years of high rainfall and except for a 1-year attenuation of Wekiva River caused by depletion of ground-water storage, so do the years of low flow and low rainfall. Streamflow may average above normal during a year following a wet year because of the carry-over of storage from the wet year even though rainfall is below normal. After a severe drought, streamflow may average below normal and even decrease during a year of above-normal rainfall because of the large amounts of water required to replenish the depleted soil moisture before an excess to provide runoff becomes available.
The flow of Wekiva River is much less variable than that of Econlockhatchee River and St. Johns River because it is maintained by the flow of large springs that discharge from a vast highly permeable ground-water reservoir (Floridan aquifer). The base flow of the Econlockhatchee and St. Johns Rivers is maintained mostly by seepage of water from the relatively thin and low yielding water-table aquifer. Their channels are very shallow so that a small drop in the water table causes a sharp reduction or cessation of ground-water inflow.
Figure 11 shows that the distribution of monthly runoff during the year corresponds in a general way to the distribution of rainfall, but there are some apparent discrepancies. Although average rainfall for months March, April, May, and October is about equal, runoffs for these months differ widely. Average runoff decreases from March to May because evaporation and transpiration losses increase during this period (See fig. 41.). Runoff for October is higher than that for May because in October evaporation is less and storage, which increased during July, August, and September,