In the late summer and early autumn of 2019, Texas experienced severe to extreme drought over portions of Central, Southern, and eastern West Texas Regions. That drought is captured in weekly map images provided by the United States Drought Monitor, through a partnership between the National Drought Mitigation Center at the University of Nebraska-Lincoln, the United States Department of Agriculture, and the National Oceanic and Atmospheric Administration.
Being able to anticipate the impact of dry weather or drought on water levels in local aquifers is critical to the long-term management of shared groundwater resources. Fundamental to these planning efforts must, therefore, be the collection of accurate, continuous, time-series water-level data that will capture periods of drought during the water year. Every drought is inherently unique in character. By collecting continuous data over many years a greater understanding is gained of the background variations in aquifer water levels – both seasonal and event-driven.
No detailed analysis of the relation between drought and groundwater levels has been attempted here – rather what follows is a simple juxtaposition of Drought Intensity against the hydrographs of wells located in areas affected by the 2019 drought. The intent is to document that juxtaposition, to highlight the importance of data collection and continued monitoring, and to encourage further inquiry.
The 2019 Texas Drought
Three selected map images (Figure 1) provide a view of the progression of the drought intensity that was at its greatest in late September 2019 and early October 2019. These maps use a color range from yellow (D0), indicating “Abnormally Dry” conditions, to brown (D4), indicating “Exceptional Drought” conditions (Figure 2). Considering the total area of Texas, < 1% of the state was experiencing Severe or Extreme Drought Intensity (D2-D3) during the week of July 23 (Figures 1 and 3). By the week of October 15, that state-wide percentage had grown to about 26% with much of the drought appearing in the Central, Southern, and eastern West Texas Regions. By the week of December 3, much of the Extreme Drought had abated and the area of Severe Drought had lessened as well – together those categories of Drought Intensity (D2-D3) had dropped to about 8% of the area of Texas (Figures 1 and 3).
Figure 1. Map images of Drought Intensity in Texas for specific weeks in 2019, accessed from U.S. Drought Monitor Map Archive on December 7, 2019.
Figure 2. Legend for map images of Drought Intensity in Texas, accessed from U.S. Drought Monitor Map Archive on December 7, 2019
Figure 3. Details of Drought Intensity in Texas for specific weeks in 2019, accessed from U.S. Drought Monitor Map Archive on December 7, 2019
Aquifers Under Drought Conditions
Drought Intensity is an interpretation – undertaken by the United States Drought Monitor – of many climatological, agricultural, and hydrologic inputs. A comparison of Drought Intensity to changing water levels in wells over any selected period of time may provide insight to the effect of drought conditions on local aquifers.
Downward trending water levels in an aquifer during a period of drought can have numerous causes, including:
- increased water withdrawals from the aquifer to meet greater dry weather needs,
- reduced or no recharge from precipitation,
- reduced or no recharge from surface water that is typically contributing to groundwater.
The shallow aquifers of the Central, Southern, and eastern West Texas Regions are quite variable in their hydrogeology – including their primary and secondary permeability – and so their response to climatic and hydrologic conditions can be very different. To understand the contributing factors to water-level changes in a specific monitoring or pumping well requires a deliberate investigation of each well or well network. No such Investigations have been conducted here, so the factors contributing to any relation between Drought Intensity and observed changes in water level in the well hydrographs presented below, remain to be explored.
Observed Water-Levels in Pumping Wells
Each of the following hydrographs present water levels measured in wells from Wellntel monitoring networks and were created using the Wellntel Analytics Dashboard. The data presented are for the period of June 1 to December 8, 2019.
Figure 4 presents hydrographs from four pumping wells located in the same residential development in Central Texas – data presented are weekly maximum water elevations. The water level in this shared aquifer trends lower in these wells through late August or so and then a partial recovery is visible.
Figure 4
The Wellntel Analytics Dashboard provides the opportunity to view the data from Figure 4 as water-level change from a common zero reference point using the “Change” feature set. Figure 5 shows this plot – Depth to Water Level Change – highlighting the change between June 1 to August 8 – to have been about 44, 47, 49, and 56ft for the four wells.
Figure 5
Figure 6 presents the hydrograph from a residential pumping well in the South Texas Region – data presented are discrete depth measurements, both timed and pumping-influenced measurements. About 20ft of water-level decline is revealed from June to early October – however, this well continues a downward trend falling another 10 ft through early December. Note that the non-pumping (static) level and depth of pumping drawdown trend lower proportionally. This is a rural area with low well density – a consideration in determining the extent to which the temporal change in water level has been influenced by local pumping versus drought impacts.
Figure 6
Not very distant from the well of Figure 6 is a well with a very different hydrograph over the same period of time – data are presented as weekly minimum depth measurements. As presented in Figure 7, water levels in this well fluctuated only around 0.5ft, but with a rising trend. Hydrogeologic factors likely are a prime influence on the difference in the observed response in these two wells.
Figure 7
Figure 8 presents a Central Texas Region water supply well – data presented are discrete depth measurements, both timed and pumping-influenced measurements. Pumping frequency appears relatively consistent throughout the period so water withdrawals may not be a substantial factor contributing to the water level decline. Note that the non-pumping level and depth of pumping drawdown generally trend lower proportionally. Water level reduction from June to October is about 10 ft with a hint of recovery into December.
Figure 8
The climatic variability of droughts and the hydrogeologic variability of aquifers contribute to the complexity of observed data sets. The collection of statistically appropriate, continuous, time-series water-level data (to support acceptable uncertainty) is foundational to unravelling the combined influence of drought, recharge, and water withdrawals on well hydrographs.
Wellntel enables groundwater-level monitoring networks composed of private, submersible pump wells. Please contact us about your needs for developing a groundwater monitoring network that will help you sustainably manage your groundwater resource through extreme weather and changing climate.
Contact Wellntel’s VP/BD Charles Dunning, PhD to learn more about Wellntel networks, groundwater science, and extreme weather.