Water Budgets

1
Water Budgets

• Key Questions
• What are the major sources of water for lakes,
  and what controls their relative importance?
• How do these sources affect lakes?
• What environmental factors control lake water
  balances?

2
The Global Water Cycle
26000 14000
3
Lake Water balance

• dVlake/dt Qin - Qout
• Q flux of water, m3/day (or /sec)
• Qin Precipitation, rivers, surface runoff,
• groundwater (subsurface runoff
• and true groundwater)
• Qout Evaporation, outflow, groundwater

4
Precipitation

• Global Rainfall from 0 to 11 m/yr
• US lt0.1 m/yr to 4.5 m/yr
• Providence 1.16 m/yr
• Importance affected by ?
• Influence on lake?

5
Water from catchment

• Lost by evapotranspiration
• Surface runoff (overland flow)
• Subsurface runoff and rivers
• Deep groundwater

6
80
30
45
100
55

• Evapotranspiration (loss from catchment)
• - From 100 to 5
• - Function of regional climate

7
Surface runoff

• Water influx exceeds soil holding capacity
• Most common in
• - Flashflood-prone regions (semi-arid zones)
• - Spring/fall over frost
• - Disturbed soils/forests
• Effects on lakes

8
River and stream inputs
USA

• Related to basin size, geology
• Measured directly with river gauges

9
Groundwater

• Unimportant in many lake
• Major source for karst lakes
• Affects lakes

10
Devils Lake, ND
11
Water Losses

• Seepage Lakes groundwater outflow
• Outflow vs. evap and open/closed basins
• Evaporation calculated (Penman or energy balance
  methods)
• Varies from 0.25 to gt 2.5 m/yr in US

12
Evaporation

• Evaporation
• Cloudiness
• Windspeed
• Humidity
• Air temperature
• Lake temperature
• Surface area

13
Lake Water balance
14
The Balance

• Atmospheric inputs balance evap.
• Catchment inputs balance outflow

15
Summary

• Global distribution of lake function of
  Precip-Evap and formation of lake basin
• Type of water input/output influences limnology
• Lakes fed by direct precipitation, surface flow,
  and groundwater
• Lakes lose water by outflow, evaporation, and
  groundwater.
• The primary factors that control the relative
  balances are catchment area, climate, and local
  geology