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University of Texas at El Paso





Low Impact Development in Desert Climates and the New Water Balance Equation

What Low Impact Development (LID) Accomplishes

Low Impact Development (LID) is typically considered a stormwater issue. Instead of ugly, wasteful, detention or retention ponds the stormwater is captured locally with bioretention basins, rain gardens, tanks, and permeable pavement. For humid climates the focus in on retention of stormwater to prevent downstream flooding and removal of contaminants from the water where appropriate.

In fact, LID has several important benefits, and the relative importance of each changes with the climate. Major benefits include:

     a) stormwater is removed near the source with a distributed system that should be more robust relative to failure,

     b) the stormwater supports the growth of vegetation that is relatively dense and lush relative to the climate norms,

     c) stormwater may be cleaned through biological processes,

     d) the need for watering of landscape plants with municipal or well water is greatly reduced or eliminated.

In humid climates a and c are most important. In arid environments b and d are most important. This is because saving water is more important in arid environments.

The New Paradigm for Urban Water Balance in Arid Environments

Over the past 25 years municipalities throughout the US but especially in the semi-arid western states have worked at ways to reduce urban water use. Techniques included reducing the prevalence of grass lawns, lawn watering restrictions, low flush toilets, low flow shower heads, low flow faucets, and reducing household demand pressure. The focus was on reducing per capita water use. Over the next 25 or more years this will change dramatically and the list of changes that actually save water will have to be revised.

Western water is predominantly allocated and new sources are few. Many supposed new water sources (e.g., new groundwater pumping) rely on incomplete/magical water balances that fail to fully account for long term changes in current groundwater discharge areas. The tight water supply, combined with tremendous technological innovations in water treatment and desalination, is gradually leading to nearly complete recycling of wastewater. The "toilet to tap" issue is sometimes dealt with rather stealthily. In El Paso, Texas for example we put wastewater briefly back into the Rio Grande then extract for use as drinking water ~1/4 mile from the outlet. At a different location wastewater is infiltrated to groundwater then extracted from nearby well for use as drinking water. Plausible deniability strategies of this type will gradually be supplanted by direct and full potable water reuse.

The largest source of unallocated water comes from development itself. Replacement of undeveloped ground with roads, driveways, parking lots, and roofs reduces the area of natural vegetation and increases the area of impervious surface. Consider that if 80% of a new subdivision consists of roads, roofs, sidewalks, and driveways, the remaining 20% of the land has access to 5 times the climatic norm of water. With careful design, this allows relatively dense landscape vegetation to be grown with no use of municipal water supply. Water rights issues can be addressed where necessary by equating pre and post development stormwater generation. If a new subdivision produces the identical volume of stormwater that the same property produced prior to development, how can in subdivision use of the excesss stormwater coming from roofs and roads be taking someone else's water rights?

Complete recycling of wastewater for potable reuse means that water going down the drain is no longer lost:

     a) low water use appliances (toilets, shower heads) no longer "save" water, (although they will save money),

     b) water is lost from the urban system whenever it is applied to landscape in a manner leading to evapotransipiration, and

     c) water is lost from the system when excess salts are added to the wastewater by appliances such as water softeners.

In the new water balance water losses are primarily from landscape water, evaporative coolers, and water softeners. Water softeners hurt because the excess salt they add to wastewater reduces the ability to recover the water. With landscape water eliminated and most wastewater recycled urban areas need relatively small quantities of water to prosper.

Proper implementation of LID allows urban landscape to take advantage of impervious surfaces to grow relatively lush landscape while eliminating the largest source of water loss. In arid environments the water savings will be increasingly more important than the stormwater control to be achieved from LID.

Arguments Against LID

Problem: Stormwater control is easier to manage with large retention ponds than is is with dispersed bioretention basins. A new development could have LID but when a house is sold the new owner may remove the bioretention basins and recontour the lot. This means that important stormwater controls would be lost. Unless we can enforce the stormwater controls we are not ready for LID.

Answer: It is unclear how urban enforcement of bioretention at lot and development levels in the age of Google Earth and other free software is difficult or expensive. More importantly, for arid regions, the water savings from LID are dramatically more important than stormwater benefits. These important benefits should begin as soon as possible. Choosing the awful option because the excellent option is not perfect is something that seems inappropriate in the states farthest from the national capital.

Problem: LID will only support non-native species like Palo Verde in El Paso.

Answer: A survey of inadvertant stormwater capture areas in the El Paso del Norte Region indicates a wide variety of native and even water hungry trees grow on their own with no artifical watering or maintenance.

LID Adapted to Desert Environments

Low Impact Development (LID) in a desert environment is challenging because one must hold and store large quantities of water between the sometimes large and usually sporadic precipitation events. This requires a careful balance between climate, capture area, water storage in the soil, and plant water use. The same problem is much less important, and thus typically ignored, in more humid climates.

A properly designed LID household, subdivision, or commercial development provides a green, shady, environment without the need for supplemental watering or sprinkler systems - even in the desert. A secondary benefit is that the LID systems reduce stormwater runoff. From a municipal standpoint the stormwater control aspects of LID may be more difficult to implement because they require the cooperation of individual property owners (difficult to enforce). However, there is nothing to prevent implementation of LID as a water saving and shady, green habitat tool, even without taking stormwater control credit for the LID features.

The linked Excel spreadsheet model performs a water balance based upon 30 years of climatic data from El Paso, Texas. The directions for the model are here. The spreadsheet assumes that the bioretention features are designed for a user specified maximum storm size. The model then calculates: a) anticipated average stormwater runoff from the lot, b) anticipated average groundwater recharge, and c) crown area of plants that can be grown in each microwatershed.