Groundwater Recharge Through Playas

Groundwater Recharge Through Playas

Playas are a primary source of groundwater recharge, contributing up to 95 percent of water flowing to the aquifer. Recharge rates in playas are 10 to 1,000 times higher than under other areas. The average rate across the region is about three inches per year — that’s three inches of water the size of the playa moving toward the aquifer each year.

The average rate across the region is about three inches per year — that’s three inches of water the size of the playa moving toward the aquifer each year.

In November 2015, we held a Playa Recharge Summit with 14 scientists and researchers who study various aspects of the playa ecosystem to determine what was known about groundwater recharge through playas. The experts agreed that while the recharge rate is not fast enough to counter the amount of withdrawals due to irrigated agriculture, it can support farmers and ranchers with rainfed (dryland) crop production or grazing systems, as well as towns and communities that depend on the aquifer.

This recharge through playas is a continuous process. In fact, water reaching the aquifer today started its journey during our parents and grandparents lifetimes; the water recharging now will be available for today’s children and future generations.

What the Scientists Say

Amount of Recharge Through Playas

Playa experts agree that playas recharge the aquifer at the rates described in the 2009 USGS Circular: Recharge Rates and Chemistry Beneath Playas Literature Review, which gathered and reviewed all the scientific literature about recharge to the High Plains Aquifer. According to the report authors, “Reported recharge rates beneath playa floors range from about 0.01 to more than 10 in./yr, whereas most interplaya settings in croplands and rangelands have recharge rates reported to range from about 0.01 to 1 in./yr.” The scientists at the Summit supported using an average of 3 inches per year across the region in calculating recharge rates through playas.

As the USGS recharge report indicates, recharge rates vary based on a number of factors including depth to aquifer, soil type, amount of soil saturation, evaporation and transpiration, and amount of rainfall. Recharge rates also vary during the playa wet/dry cycle. According to the USGS report, most recharge occurs through the large cracks that form in the dry, clay soils in the bottom of the playa. During a rainstorm, water runs off the landscape and into the cracks, eventually making its way to the aquifer. Then when soils become saturated and the cracks close, water continues to seep into the soil and slowly moves downward along roots and small channels, as well as along the playa’s perimeter where the clay soils meet the upland soils. Recharge during this time happens at a much lower rate.

The USGS recharge report concludes that “properly functioning playa wetlands … are thus important for the overall recharge contribution to the southern High Plains aquifer.”

Time to Recharge Through Playas

In general, water recharging today through playas will be available for our children and/or grandchildren. The time it takes for recharge to reach the aquifer and be available for extraction varies depending on depth to the aquifer formation and underlying soil type. If it is shallow, water will recharge in months to years. If the depth is approximately 100 feet, it will take years to decades. If the depth is approximately 200 feet, water will recharge in decades. At the deepest locations, recharge could take a century to reach the aquifer.

Quality of Recharge Through Playas

The benefit of a healthy playa — a playa with a grass buffer and no hydrological modifications such as pits or ditches — goes beyond simple recharge. Research shows that water reaching the aquifer through playas is of higher quality than that going through other pathways. This happens in two ways: first, as rainfall and runoff travel toward the playa, the surrounding grasses trap sediments, which can carry contaminants into the playa; then, as the water moves through the clay floor of the playa, a second ‘cleaning’ process occurs as the soils beneath the playa remove nitrates and other dissolved contaminants. The result is high quality water reaching the aquifer that can then be used by those living on the land.

Direct Recharge Benefits to Landowners

Once the water reaches the aquifer, it is yours to use. Unlike a lake, the aquifer is made up of many smaller sections with extremely slow water flow between sections (1⁄2-1 mile per 10 years). Thus, the water is functionally compartmentalized in a given area. That means the water recharging through your playa benefits you directly and provides high quality water that can be used by you, your children, and future generations.