Water Balance

Generally, water budgets are presented in a static mode representing typical conditions. At times, this leads to misconceived ideas about response of the system to future stresses. It is important to consider the dynamics of the system to avoid pitfalls such as dividing the volume of water filling pore spaces by the usage rate to estimate the period of time that current usage could be sustained.

Simplified Hydrologic System in a Front-Range Aquifer

Simplified Diagram and 3D rendering

Consider the dynamics: when pumping increases, water levels decline, the water table gradient decreases, base flow decreases, thus stream flow and ground-water outflow decrease (see diagram above). If a drought occurs precipitation (PPT) and evapotranspiration (ET) decrease, but not proportionately (precipitation decreases more than ET (see Table below) because drought conditions increase evaporation and use of the little water that is available so the percent of PPT that is ET increases, consequently water levels, stream flow and ground water outflow decline and their percent of PPT decreases.

In the extreme, if precipitation ceases for a long period, all the water in the pores of the above diagrams is not available for use. Rather, groundwater continues to flow from the basin whether wells are pumped or not and ground water levels decline. Water levels do not decline uniformly, rather they drop more rapidly at higher elevations such that the gradient decreases with time and thus the rate of base flow to streams and ground water flow out of the basin decrease exponentially as observed in typical recessions in arid basins each summer. The rate of decline depends on the transmissivity and storativity of the rocks and soils.

Development of a sustainable system involves consideration of the dynamic balance. At the extreme, an unsustainable system is one in which average water consumption exceeds average recharge. Water levels will decline with occasional short-term temporary increases following recharge-events and longer-term temporary increases if average consumption does not exceed recharge in wet years. For example, if average recharge is 1in/yr, it may range from 0.75 to 1.25. If average consumption is equivalent to 1.1in/yr spread over the basin, the basin will eventually go dry, although water levels will rise in years when recharge is 1.25in/yr.

A sustainable condition requires that average consumption is less than the average recharge (see Figures below). In such a case, a new equilibrium condition will be established with lower water levels and a lower gradient such that the average ET, and discharge to stream base flow and ground water outflow is reduced. This may be acceptable so long as the total stream flow is sufficient to serve downstream users and maintain an acceptable environment for aquatic species in the stream, and ground water outflow is sufficient to maintain acceptable water levels outside of the basin in areas that receive ground water inflow form the basin.

Above is a section of a basin with parameters similar to TCB showing equilibrium hydraulic heads for an average recharge rate [equivalent to 1inch/yr evenly distributed over the basin]

Above is the same section with 50% of average recharge consumed by development, causing head declines on the order of 250ft (76m) as controlled by the geometry, transmissivity and recharge rate. Assuming instantaneous development to this level, the system reaches the new equilibrium in 1.6 years, as controlled by the storage coefficient (0.02 is used in this case)

Above is the same section with 10% of current recharge, which nearly dries out the basin.

The challenge is to determine 1) average recharge to gage the level of sustainable consumption and 2) drought probabilities, as well as transmissivity and storage coefficient to assess how much below average recharge we must design for, and how deep wells must be, in order to sustain the community through drought. The largest water level declines will occur at wells because they are discharge points. Consequently average water level declines are not sufficient to determine whether wells will go dry, local scale analysis is necessary for that purpose.

REFERENCES

USGS, 2002, David Stannard, personal communication

USGS, 2001, MGWRS Phase I Report Summary, Water resources assessment of the Turkey Creek Watershed, 1998 to 2000, prepared for Jefferson County Planning and Zoning Department, by USGS Colorado District, 18 p