A Study of Temporal Variations of Groundwater Level, Recharge, and Baseflow: Does Scaling Exist in These Processes?

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0510322ZhangGroundwater recharge (R) from precipitation and discharge to rivers as baseflow (BF) are two of the mainprocesses in the basin-scale water cycle. These processes vary with time and space. Effective managementof water resources requires deep understanding of these processes. Due to the lack of data or thedifficulties in measuring R and BF, however, our current knowledge and understanding of these complexprocesses are very limited, especially about their temporal and spatial variations. Various assumptions aremade about spatial variations of R in existing studies and little attention has been given to temporalvariations of R and BF. While it is difficult to measure R and BF, it is relatively easy to measure the waterlevels in observation wells (h) and runoff in rivers. Extensive groundwater level data sampled at varioustime intervals over periods of years or decades are often available. Fluctuations of the water table in agroundwater system are dynamic responses of the system to its recharge and discharge, and thus containsignificant amount of information about the recharge and discharge. Our preliminary analyses of theobserved water levels in seven monitoring wells at the Walnut Creek watershed of Iowa indicate thatfluctuations of the groundwater levels is a temporal fractal whose fractal dimension (D) varies spatially.Baseflow from this and other four watersheds also behaves as a temporal fractal and, more importantly,has a transition time or break in scaling.Our objectives are: 1) to determine existence of fractal scaling of groundwater level and baseflow andtheir relationship with recharge; 2) to identify physical causes behind fractal scaling; 3) to study the effectsof aquifers' physical heterogeneity on fluctuations of groundwater level, recharge, and discharge and ontheir scaling, and 4) to investigate the nature of groundwater recharge process. These objectives will bereached by testing six hypotheses. These hypotheses will be tested by collecting and analyzing the longtermgroundwater level measurements in 42 monitoring wells in 6 groundwater regions and thestreamflow at 58 gauge stations in 11 hydrological units over a period of 50 - 100 years at the USGGwebsite, and by conducting an integrated hydrologic modeling (IHM), stochastic analyses, and numericalsimulations. With data mining we will use the extensive data collected by USGS over the years to identifythe existence of the temporal scaling. With IHM we will be able to assess impacts of temporal and spatialvariations of P, ET, and ? on fluctuations of h and BF. With stochastic analyses and numericalsimulations, we will investigate effects of spatial variations of aquifers' physical properties on thetemporal variations of h and BF and their scaling as well as the nature of R process.The intellectual merits. The most important merits of this research are to find the temporal scaling of hand BF and to provide physical causes behind the scaling. Temporal scaling of h may be pervasive inmany aquifers as we show in this proposal by the spectra of the longest water level data in 14 USGS wells.There may be a simple explanation for the scaling: fluctuations of groundwater levels are due to variouscontributing hydrological variables (e.g., P, ET, and ?) with differing time and/or spatial scales. As aresult, there is no characteristic time scale in the head fluctuations! We are anxious to do furtherinvestigation. Whether processes in the natural world are dependent or independent of the scale at whichthey operate is one of the major issues in hydrologic science . (Sposito, 1998). There has been a significanteffort in searching for scaling in hydrology since an invariance property across scales as a fundamentalshould guide data analysis and modeling methods (NRC, 1991). Significant progresses in searching spatialscale invariance have been made during the last decade in hydrology. However, less attention was given totemporal scaling of subsurface hydrological variables. We have proposed a relatively new direction ofstochastic research, as summarized by the panel on the original proposal, and are trying to make a forwardstep in application of stochastic approaches. The broad impacts include: 1) use of abundant groundwaterlevel and streamflow data in trying to understand the patterns of temporal variations of h and BF make themethods and results of the proposed research widely applicable, 2) the proposed research is closely relatedto the NSF initiative: Water Cycle Research, 3) the knowledge obtained in this research is needed in longtermmanagement and planning of water resources and in dealing with climate changes, 4) A current Ph.D.student is working on this topic. Two new students (one graduate and one undergraduate) will participatein this project if the proposal is funded, 5) Several publications are expected from this research as twopapers have been written (one published and another submitted recently) based on our preliminary results.Finally, we want to say that addressing the comments made by the reviewers and panel on the original andsecond-submission of this proposal has greatly improved the proposal.