Across the West, rapid-fire residential and commercial development is setting
off five-bell alarms among water users. Here in Montana, where land-use planning
is a four-letter word, irrigated agricultural land is making way
for residential development at breakneck speed, especially near booming urban
centers like Bozeman, Billings, Kalispell and Missoula.
In the face of uncontrolled change, senior water-rights holders and instream flow advocates are joining forces to stave off well-financed developers near the final stages of their projects. Their gut-level reactions are inherently incapable of implementing viable alternatives based on hydrologic reality. Theyre too little, too late.
Testifying recently before the Montana House Natural Resources Committee, I was shocked by the uninformed questions asked by committee members. Lacking the most basic understanding of cause-and-effect relationships in hydrology, how can lawmakers be expected to create a sound legal framework for managing water resources?
Conversion from irrigated agriculture to residential and commercial development is altering the seasonal flow patterns to which we have grown accustomed. Although the timing and scale of change varies from place to place, the general pattern is predictable. The challenge for legislators and land-use planners is to understand these patterns and to make conscious, informed decisions about whether to accommodate or mitigate them.
The change from irrigated agriculture to residential development entails more than simply pumping groundwater. Most irrigation systems in the West especially the oldest systems on the most productive ground use diverted surface water. Irrigation water that crops do not use seeps into the soil and eventually reaches the water table, where it recharges groundwater in the underlying aquifer. So-called irrigation return flow is a major source of groundwater recharge in irrigated western valleys.
The irrigation-charged groundwater slowly makes its way underground to rivers, streams and springs, where it eventually discharges. Groundwater discharge from irrigation return flow keeps rivers flowing well into late summer and fall, even after all the snow has long since melted, even after the rains have stopped. Although not a natural phenomenon, we consider this annual flow pattern normal, for it has recurred for more than 30 years.
The most important hydrologic change brought on by urban and suburban development is a drastic reduction in groundwater recharge. Urban land surfaces such as roofs, roads and parking lots are impermeable. Rain and snowmelt run off these surfaces, instead of seeping into the ground and recharging aquifers. In a typical engineering design, runoff is quickly shunted into the nearest stream or river to rid the area of potential flood waters. Consequently, localized recharge greatly decreases, streamflow becomes flashier (larger fluctuations over shorter periods of time), and late-season, groundwater-fed streamflow decreases. When irrigation stops, seepage from excess irrigation water also stops, or continues to recharge the aquifer only from leaky ditches.
Almost without exception, rural residential development in the West relies on well water for domestic use. So, on top of reducing aquifer recharge, the change from surface-water-irrigated cropland to groundwater-irrigated yards increases aquifer discharge. Less water goes into the aquifer than before, and more water goes out.
Previously, irrigation diversions depleted streamflow in the spring and early summer, and irrigation return flow maintained streamflow well into the late summer and fall. Now, with fewer surface-water diversions, early flows increase, as does the risk of flooding. Conversely, late-season flows decrease, potentially leaving fish and downstream irrigators high and dry.
Overall, urban and suburban development consumes less water than cropland. So, in a sense, development returns the hydrologic system to a more natural state. But if policy-makers decide to accept this change, it needs to be a conscious, informed decision, not the default consequence of head-in-the-sand avoidance. The discussion then needs to turn to compensating existing water-rights holders who will lose their late-season irrigation water.
Opponents of land-use planning argue that we do not understand hydrologic systems sufficiently to predict or mitigate impacts of development. Granted, detailed knowledge is needed to predict precise locations and timing of effects. But whether the water table drops 10 feet or 50 feet, or whether streamflow depletion peaks in August or September, basic tools to mitigate these impacts are the same.
For example, instead of shunting rain and snowmelt away from subdivisions, planning departments could encourage onsite infiltration. Where the soil does not permit infiltration, storm runoff and treated wastewater could be injected underground.
When sewers were put in place in Long Island, N.Y., in the 1950s, wastewater that previously recharged the aquifer now discharges straight into the ocean. The loss of aquifer recharge caused the water table to drop about 20 feet. To save the aquifer, more than 3,000 small recharge basins were constructed. Their average combined infiltration rate of 150 millions gallons per day has successfully reversed the trend of declining water levels in the aquifer.
Out West, many creative options exist for water management. Most of the basins within the Basin and Range province, which, loosely defined, extends from Canada to Mexico, provide ideal geologic settings for storing artificially recharged water underground. Using existing irrigation infrastructure, we could spread spring runoff onto benchlands, allowing it to flow underground toward rivers, where it would replace irrigation return flow as a resource for late-season use. Another simple option is to discourage landscaping that requires irrigation.
Maintaining current streamflow patterns in the wake of land-use change requires preemptive engineering. Regardless of which approach is chosen, basic hydrologic principles are guidance enough to begin the process of informed decision-making and water-management planning.