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Farmers should track aquifer saturated thickness to plan crops and irrigation

Groundwater-dependent growers should establish monitoring networks early in the water year to quantify saturated thickness before the growing season.

A direct relation exists between the saturated thickness of an aquifer and the groundwater volume available for extraction constrained by regulation and/or principles of sustainability.

Under natural conditions, the water table of an unconfined aquifer will rise and fall as a function of recharge, discharge, and evapotranspiration. Saturated thickness of the groundwater resource – defined in simplest terms as the distance between the water table and the base of the aquifer (Figure 1) – will increase and decrease in response to the fluctuating water table.

The changes in saturated thickness of an aquifer will usually be observed to repeat seasonally within a historical range driven by typical changes in those same factors – recharge, discharge, and evapotranspiration – but can also reflect longer-term climatic shifts. Imposed on top of these natural fluctuations will be changes caused by any irrigation pumping, managed recharge, or other influences by man.

Figure 1. Figure depicting the saturated thickness of a simple unconfined aquifer. Modified from StudyBlue, 2020

Because the saturated thickness of an aquifer is directly related to the groundwater volume available for extraction, quantifying the saturated thickness at the beginning of the growing season is critical to irrigation planning and perhaps even to crop selection. Key to quantifying the dynamically changing saturated thickness of an aquifer is, of course, closely monitoring the transient depth to the water table.

Hydrologic observations are usually considered within the framework of the Water Year, which is defined as the 12-month period beginning October 1 of a given year and ending on September 30 of the following year. Establishing a new groundwater-level monitoring network – or expanding an existing network – will increase the understanding of the short- and long-term fluctuations of the water table. By adding these monitoring points early in the water year, saturated thickness can be quantified with greater spatial and temporal confidence heading into the irrigating season. 

The following examples illustrate fluctuations in the water table – and by extension aquifer saturated thickness – observed in wells in California and Texas. 

The first is a hydrograph of a water-supply well at a California winery, illustrating fluctuations of the water table influenced by pumping of the well and by seasonal hydrologic conditions (Figure 2). This hydrograph makes clear to the vineyard operator that the saturated thickness of this groundwater resource was about 15 feet less at the beginning of Water Year 2020 than it was at the beginning of Water Year 2019, and changes to irrigation operations may be warranted.

Figure 2. Water-level elevation data collected by a Wellntel system from a water supply well at a California vineyard, data include pumping and non-pumping levels that span parts of three water years.

A second example is from the Ogallala Aquifer in Gray County within the Llano Estacado Region of Texas – presenting a well with longer records of water-level data collected by the Texas Water Development Board and reported by John E. Stout, of the USDA-ARS (Stout, 2018). The hydrograph of the recorder well provides data from 2005 through 2017, clearly showing the influence of seasonal pumping by irrigation wells neighboring the monitoring well (Figure 3). It is noted that the depth to water at the start of seasonal pumping in Water Years 2014 and 2015 is clearly greater compared to Water Years 2005 and 2006. And even though it appears the difference is less than one meter, such a reduction in saturated thickness and available groundwater may influence local agriculture operations.

Figure 3. Multi-year depth-to-water hydrograph for the recorder well in Gray County, Texas, water-level data collected by staff of the Groundwater Monitoring Program of the Texas Water Development Board. Modified from Stout (2017).

These data from the Llano Estacado Region of Texas illustrate the value of long-term data collection and in particular the value of quantifying the saturated thickness of the groundwater resource at the beginning of the water year to enable better and more informed agricultural water management of the Ogallala Aquifer.

Whether the groundwater volume available for extraction is constrained by regulation or by principles of sustainability, is it critical for private well owners to have data and information about the changing saturated thickness of their local aquifer. Wellntel enables groundwater-level monitoring networks composed of private, submersible pump wells, as well as monitoring wells, and provides an Analytics Dashboard with powerful tools for data management, analytics, and visualization.

Contact Wellntel’s VP/BD Charles Dunning, PhD, to learn more about Wellntel’s Monitoring Networks and to explore possibilities for your monitoring objectives. 

Cited references:

Stout, J.E., 2018, Seasonal water-level perturbations beneath the high plains of the Llano Estacado; Journal of Hydrology: Regional Studies, Volume 18, August 2018, accessed January 18, 2020 at https://www.sciencedirect.com/science/article/pii/S2214581817303373

StudyBlue crowdsourced library, access January 26, 2020 https://www.studyblue.com/?src_url=https://www.studyblue.com/notes/note/n/water-beneath-the-surface-ch-6-3/deck/13436902#flashcard/flip/13436902