The Lower Mississippi River Alluvial Valley is a floodplain that spans seven states. It is suffering from groundwater depletion – a long-term water level decline due to human use. Irrigation and overuse of water resources have led to a seven meter drop in groundwater levels from 1987 to 2014. Water from precipitation and other sources naturally makes its way into groundwater – in a process known as groundwater recharge. Groundwater recharge is known to vary between agricultural and forest land.
USDA Forest Service researchers Ying Ouyang, Johnny Grace III, Wayne Zipperer, and their collaborators set out to determine the effects of land use on groundwater recharge in the Lower Mississippi River Alluvial Valley. Their results were published in the Journal of Hydrology: Regional Studies.
Management strategies implemented in the region to prevent and reverse the drop in groundwater levels include the use of water retention ponds for irrigation as well as afforestation of unproductive farming land. But studies in semi-arid regions suggest forestland is less effective at groundwater recharge than agricultural land. This study is the first of its kind in a humid subtropical region like the Lower Mississippi River Alluvial Valley. The region is characterized by its hot and humid summers.
Because a direct measurement of water recharge is challenging, the researchers used a modeling approach and focused their efforts on the Lower Yazoo River Watershed. They chose this location because it is representative of a typical watershed in the region. It is about 61 percent forest land, 31 percent agricultural land, and 4 percent wetland.
The researchers simulated groundwater recharge in the various land use types over a ten-year period from 2000 to 2009. With the model, they were able to simulate many factors affecting groundwater recharge: precipitation, water runoff, precipitation interception, and evapotranspiration – the process by which water moves into the atmosphere. All these forms of water movement affect how much water from precipitation ends up as groundwater.
The researchers found that throughout the watershed, active groundwater recharge was ten times higher than inactive groundwater recharge. “Active groundwater is shallow groundwater that has connections to streams and rivers,” says Ouyang, adding that “inactive groundwater is very deep and does not interact with moving bodies of water.”
The slow recharge into inactive groundwater is believed to be caused by the depth of the aquifer, which is about 50 meters below the surface, and the presence of a thick clay layer which makes water movement into the aquifer difficult.
The scientists also found that more rain does not always lead to more water runoff. The amount of annual water runoff was highly dependent on precipitation from the previous year. When soils had low water levels due to lower past precipitation, runoff could also be low – even when the amount of precipitation more than doubled.
In the Lower Yazoo River Watershed, the researchers found that both active and inactive groundwater recharge was higher in forested land than in agricultural land – contradicting the findings of previous studies analyzing the groundwater recharge differences between these land use types. The discrepancy is possibly due to the differences in climate. Groundwater recharge in humid subtropical settings was largely unexplored before Ouyang’s study.
As for other groundwater depletion mitigation strategies, Ouyang points out that we “can improve irrigation efficiency to make sure that irrigation only occurs when crops need water, or build water retention ponds to use pondwater for crop irrigation.”
For more information, email Ying Ouyang at firstname.lastname@example.org