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. They’re
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.
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