Drought in Groundwater Dependent Systems Implications of Climate Change?

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1 Drought in Groundwater Dependent Systems Implications of Climate Change? Sreeram Singaraju., Ph.D Research Associate Engineer Water Resources Center Texas Tech University Venkatesh Uddameri., Ph.D (Co-Author) Elma Annette Hernandez., Ph.D (Co-Author)

2 Groundwater and Droughts Groundwater resources act as a buffer against droughts Not affected as much by climatic shifts Signal can be attenuated Readily accessible Not as regulated as surface water There is generally a greater reliance on groundwater during droughts How do Groundwater Systems Respond to Droughts? Karst Aquifers are more Sensitive to Droughts 2

3 Edwards Aquifer The Perfect Location Lack of significant surface water Growing Population Drought prone region Competition between economics and ecology

4 Drought Indicator Historical Precipitation data from Climate Research Unit (CRU) 4 ecologically relevant locations SPI used as the drought indicator Widely used SPI 3 Short-term SPI 12 Long-term Hydrologic Drought J-17 Well in San Antonio, TX J-27 Well near Uvalde, TX Comal Springs near New Braunfels, TX San Marcos Springs near San Marcos, TX SPI was calculated by fitting gamma distribution for each rolling sum of the month separately and then transformed using standard normal distribution 4

5 Drought Indicator Historical DOR is used for Water Planning in Texas Historical DOR 5

6 Drought Analysis Droughts were analyzed using 3 indicators Severity Sum of SPI for a given drought event Duration Number of months for a given drought event Inter-drought duration Time between two drought events Bivariate copula theory was used to evaluate the joint distribution between severity and duration 6

7 Bivariate Drought Severity-Duration Relationship Joint Exceedance Probability PP(SS > ss, DD > dd) Percentile Duration Severity Joint Exceedance 10% % % % % Measure of risk of a drought of at least a certain duration and intensity Couplas can also be used to quantify other probabilities and risks 7

8 Relationship between Drought and Groundwater Hydrology How do the water levels and springflow change under drought?

9 SPI-Water Levels in J-17 Index Well Statistically significant relationship Slope has a p-value of < 2E-16 SPI12 explains 46% of variability in water levels Drop in 1 unit of SPI relates to a drop of 10 feet of 10-day Moving Average Water Levels (At least 1 Critical Period Stage) 9

10 SPI-Comal Springs Flow Statistically significant relationship Slope has a p-value of < 2E-16 SPI12 explains ~49% of variability in spring flows Drop in 1 unit of SPI relates to a drop of 64 cfs of 10-day Moving Average Water Level (At least 1 Critical Period Stage change) 10

11 SPI San Marcos Flow Statistically significant relationship Slope has a p-value of < 2E-16 SPI12 explains ~57% of variability in spring flows Drop in 1 unit of SPI relates to a drop of 47 cfs of 10-day Moving Average flows (At least 1 Critical Period Stage change) 11

12 SPI Water Levels in J27 Index Well Statistically significant relationship Slope has a p-value of < 2.58E-12 SPI12 explains ~9% of variability in spring flows Drop in 1 unit of SPI relates to a drop of 3.25 ft of 10-day Moving Average flows (At least 1 Critical Period Stage change) 12

13 Climate Change and Droughts in Edwards Aquifer How does projected climate change impacts droughts and hydrology of the Edwards Aquifer?

14 General Climate Models Downscaled precipitation data from 231 CMIP 5 model runs from 36 different GCMs were evaluated (Data from LLNL) RCP Minimum Annual Precipitation < mm (20 th Century historical Minimum) Minimum Annual Precipitation > mm RCP RCP RCP RCP The median annual rainfall was noted to be ~ the historical median for all models 14

15 GCM Extreme Value Analysis 3 different models were selected to assess worst-case drought risks Minimum annual precipitation was less than historical (20 th century minimum) At least 1 run for all 4 RCPs GFDL Model showed highest variability across all GCM models 15

16 Historical Vs GCM Droughts will be less frequent IDD Droughts will last longer Duration Severity > drought of record is anticipated Severity Many models do predict opposite results

17 Risk of Extreme Severity and Duration Only MIROC and GFDL models indicate a higher risk of exceeding Historical 50 th percentile of Historical Severity and Duration (P(S > s, D > d)) Greater congruence among models in predicting increased risks of exceeding historical 75 th and 90 th percentile 17

18 Final Thoughts Hydrologic indicators in the Edwards Aquifer are correlated to drought Effects are most pronounced for spring flows and water level at J-17 well Not so pronounced at J-27 Uvalde well Periodic pumping from nearby pumping wells A decrease of 1 unit of SPI causes the trigger to increase by at least 1 critical stage Drier futures would place greater restrictions on water use 18

19 Final Thoughts Worst case analysis indicates that droughts will be less frequent but will be of stronger intensity and longer duration Historical drought of record may not serve as a good indicator of worst case scenario More likely to be in more restrictive Stages 3-5 for a longer period of time than Stages 1-2 Variability among GCM s pose challenges to stakeholder driven planning 19

20 Acknowledgements Texas Tech University Office of vice president for research 20

21 Questions?? 21