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

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.

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.

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.

Large-Scale Models Based on “mean-field” approaches.

Ecological patchiness is smoothed Advantages:

Tractability Few parameters Clear interpretation

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

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)

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

Problems with Quantification “Examples of indicators for a nutrient TMDL

include total phosphorus concentration, total nitrogen concentration, chlorophyll concentration, 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.”

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.

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

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.

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.

2) Field and Laboratory Studies To determine specific lake/reservoir

responses following manipulation. Replication and control. Based upon logical findings and

observations.

INSERT CORING PIC

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.

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.

Questions