HYDROLOGY AND WATER MANAGEMENT

Ed MaurerDept. of Civil and Environmental EngineeringUniv. of Washington

FUNDAMENTAL QUESTIONS(FROM AN ENGINEERING PERSPECTIVE AT

LEAST)

•How much water is there?•How much water is needed?•What are the effects of water development?

Hydrologic cycle

Quantifying Water Supply - 1

USGS Gauge network provides daily measurements

Problems: missing data, don’t account for upstream conditions, errors at high flows

Long Term Variability

Case: The Colorado River

•18 years of data (1904-1922) showed 16-18 MAF•Long term records: 13.5 MAF and highly variable

Source: U.S. Bureau of reclamation.

A history of the PDO

warm coolwarm

A history of ENSO

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Pacific Decadal Oscillation El Niño Southern Oscillation

150000

200000

250000

300000

350000

400000

450000

190

0

191

0

192

0

193

0

194

0

195

0

196

0

197

0

198

0

199

0

200

0

Ap

r-S

ept F

low

(cfs

)

Effects of the PDO and ENSO on Columbia River Summer Streamflows

Cool CoolWarm Warm

Climate Change: Changes to Snow Extent and Naturalized Streamflow at The Dalles

Current

~2045

April 1 Snow Extent

20th CenturyNatural Flows

Estimated Range of Natural FlowWith 2040’s Warming

Depletions and “Natural” Flow

Colorado River below Hoover Dam, 1934-1970

Variability and Flow ReducedAverage Natural: 20.2 kcfsAverage Historic: 13.6 kcfs

Determining Natural Flow from Gauge Observations

1. Begin with historic measurements2. Account for timing changes due to

impoundment in reservoirs3. Add in irrigation diversion4. Subtract return flows from irrigation5. Add in evaporation from reservoirs6. This requires cooperation from

historians, agricultural experts, hydrologists, and others.

Quantifying Water Supply - 2

Where gauges aren’t available: use models

This has advantages such as:•Examine effects of past development (logging, roads)•Hypothetical case of return to natural (undisturbed) state•Evaluate future trends

Quantifying Water Supply -- Summary

To describe water supply we need to understand:•Hydrologic interactions between climate/weather, soil moisture, vegetation, snow, and streamflow•Slowly-varying signals in climate•Effects of watershed diversions and impoundments

Quantifying water needs

•Domestic, Municipal, Industrial

•Instream Flow

•Irrigation

Effects of Water DevelopmentLong-term effects can be unanticipated

• Excessive pumping of individual wells locally lowers the water table and creates a cone of depression around the well

• Subsidence may occur when water is withdrawn from aquifers in unconsolidated materials causing pore spaces to collapse

• Groundwater pollution may come from single point sources or distributed non-point sources including agricultural fertilizers, landfills, oil wells, mines, septic tanks, road salt and underground

Long-Term Pumping Effects

Santa Cruz River, Arizona

Groundwater pumping near the coast

Inland pumping can result in contamination of water wells near the coast with salt water

Large-scale water development

Multi-use reservoir on Tohono O’odham reservation, ArizonaRecreation, fish, wildlife, irrigation, groundwater rechargeWon Corps of Engineers distinguished design award, 1974

Actual ProjectMax. possible water height: 75 ft.

Peak level, 1974-2000: 15 ft.

Only gauged since 1998.

Design based on downstream measurements from 1954

Ephemeral flow difficult to measure

Seepage is difficult to predict

Diversions and Stream FlowSkokomish R.Dams built 1926-30

Simple Interaction of Diversions and Downstream Uses

0

200

400

600

800

1000

1200

1400

1600

1 2 3 4 5 6 7 8 9 10 11 12

0

0.002

0.004

0.006

0.008

0.01

0.012

0.014

0.016flow

agricultural

Semi-hypothetical example using the S.F. Skokomish

System ModelMore complicated interactions and feedbacks of diversions and benefits.

Evaluate consequences of decisions using economic values of uses

Source: S. Lansing, U. Arizona.

Conclusions

•An understanding of hydrology is essential for quantifying water supplies and determining water needs.

•To satisfy water needs (and put water rights to use), priorities of uses must be established (instream/agricultural)

•Careful study of the interaction of different uses is needed to avoid conflicts

•Even carefully studied projects can have unanticipated consequences.

Timing in Water Requirements

•Phase shift between crop water (evapotranspiration) and stream flow

•Water requirement not met by precipitation is fed by diversion

•If diversion is not available, storage is needed.

•Reliability determined by variability of flows and size of storage

Precip.Evap..

Flow.