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