Reservoir Limnology of Arid Regions: Problems with Predictability.

1. Reservoir Limnology of Arid Regions: Problems with Predictability. David Walker Ph.D. University of Arizona

2. Compared to North-Temperate Regions. • Increased drainage area size. • Flashy hydrology. • Watersheds prone to increased disturbance. • Elevation gradients. • Wildland Fire. • More productive with fewer bio-available nutrients.

5. More Prone to the Effects of Climate Change • Thinner snowpacks. • Relicts from last Pleistocene will dwindle. • Increased frequency and intensity of erosive events. • Increased nutrient loading in aging reservoirs.

6. From Meta- to Micro- Scale. • Phytoplankton assemblages are extraordinarily patchy. • Overall biomass is difficult enough to model! • Knowing what assemblage types to expect under any given set of conditions is extraordinarily difficult.

8. Large-Scale Models • Based on “mean-field” approaches. • Ecological patchiness is smoothed • Advantages: • Tractability • Few parameters • Clear interpretation

9. Disadvantages • Patchiness is smoothed out(!) • Micro-scale dynamics are impossible to ascertain. • Yet these are often the metrics we are most concerned about. • The effect of the multitude of possible disturbance types is difficult to determine

10. Commonly Used Lake/Reservoir Models for TMDL Development • Watershed Models • Loading Simulation Program in C++ (LSPC) • Watershed Assessment Model (WAMview) • Storm Water Management Model (SWMM)

11. Receiving Water Models • A Dynamic One-Dimensional Model of Hydrodynamics and Water Quality (EPDriv1) • Stream Water Quality Model (Qual2K) • Conservational Channel Evolution and Pollutant Transport System (CONCEPTS) • Environmental Fluid Dynamics Code (EFDC) • Water Quality Analysis Simulation Program (WASP) • EUTROMOD • BATHTUB

12. Problems with Quantification • “Examples of indicators for a nutrient TMDL include total phosphorus concentration, total nitrogen concentration, chlorophyllconcentration, algal biomass, and percent macrophyte coverage.” • Target values for indicators then need to be established. • “Although such discrete impaired and unimpaired cutoffs do not exist in natural systems, quantifiable goals nevertheless are a necessary component of TMDLs.”

13. Square Pegs and Round Holes • Often nebulous correlations between nutrient concentrations and both type and amount of phytoplankton…or any indicator for that matter. • We simply do not know all the environmental requirements for any given species of alga to grow and survive. • Let alone interactions between species.

14. Although models are able to predict pollutant concentrations and movement with decent accuracy, they often fail completely at determining the biological response.

15. Lake and Reservoir Characterization is Difficult • All are unique • Spatial and temporal variability • Biotic interactions the we cannot (yet) detect • Nebulous correlation between environmental change, disturbance, etc. and biotic response.

16. A Three-Pronged Approach • 1)Coordinated monitoring and sampling. • Taking spatial and temporal variability into account. • Covers the “uniqueness” of individual areas. • Helps to understand the biotic response to environmental conditions. • Needs to be on-going.

18. 2)Field and Laboratory Studies • To determine specific lake/reservoir responses following manipulation. • Replication and control. • Based upon logical findings and observations.

21. INSERT CORING PIC

22. Constant Model Calibration • Data collected during monitoring and field/laboratory studies used to determine individual reservoir response to a wide variety of environmental conditions. • Models individualized. • Model refinement should be on-going. • Heuristic.

23. A Narrative Approach • Square Pegs and Round Holes • We cannot quantify the, as of yet, un-quantifiable. • Defining reservoir condition should include a combination of quantitative, semi-quantitative, and qualitative approaches. • A multi-tiered approach is the one most grounded in reality.

24. Questions