FLUVIAL FLUVIAL PROCESSESPROCESSES

J. David Rogers

Part 1Part 1

THE WATER THE WATER CYCLE CYCLE

andand

WATER BALANCEWATER BALANCE

• Concepts of the hydrologic cycle, upon which water balance assessments are normally made

WATER BALANCE EQUATIONWATER BALANCE EQUATION

P = R + I + If + ET

P = Precipitation. Most precipitation occurs as rain, but also includes snow, hail, fog drip, and sleet.

R = Surface runoff (water) from rainfall, snowmelt, or other sources, which flows over the land surface

I = Infiltration is that portion of the precipitation and runoff which infiltrates into the vadose zone, below the ground surface or stream bed

If = Interflow is the name given to lateral flow within the vadose zone which eventually exfiltrates as spring flow

ET = Evapotranspiration is the sum of evaporation and plant transpiration into the atmosphere

• Groundwater withdrawal can interrupt the natural water balance, causing water tables to lower if more water is withdrawal exceeds recharge

• A typical glimpse of water balance factors, without specifying interflow or transpiration losses

• Potential Evapotranspiration loss for the continental United States on November 15, 2006, in units of 0.01 inches of water.

• A typical plot of mean annual precipitation versus potential evapotranspiration, in this case, from central Michigan. These plots are useful for gauging irrigation demands

Vadose ZoneVadose Zone(Unsaturated (Unsaturated

Zone)Zone)• The zone between

land surface and the water table within which the moisture content is less than saturation (except in the capillary fringe) and pressure is less than atmospheric. Soil pore space also typically contains air or other gases. The capillary fringe is included in the vadose zone.

Stage Stage ChangesChanges

• When precipitation and channel flow increase, this is known as a rising stage

• When this occurs groundwater seeps into the channel, creating an influent condition, sketched at left.

• Desert species even suffer transpiration losses. Xerophytes and Phreatophytes are contrasting species of vegetation in arid regions. The term xerophyte describes a plant that has structural and physiological adaptations which enable them to survive, or even thrive, in areas with very little free moisture. Desert phreatophytes are a complex group of species with varied adaptive mechanisms to tolerate or avoid drought. They have the ability to tap a falling water table.