Water Abstractions (Week2) (copy)

Interception

• rainfall trapped on leaves and other vegetative surfaces

• trapped water doesn’t reach ground surface, subsequently lost in atmosphere

Throughfall or Stemflow

rainfall that drops through vegetation or rainfall that got intercepted but eventually will drop to ground

Coniferous Trees

• group of seed plants, a subset of gymnosperms. They are mainly evergreen trees with a regular branching pattern, reproducing with male and female cones, usually on the same tree. They are wind-pollinated and the seeds are usually dispersed by the wind.

• Intercepts water more

Deciduous Trees

• plants that seasonally shed their leaves, typically in autumn (fall) in temperate climates, or during dry seasons in tropical areas, as a survival mechanism to conserve water and energy.

• Intercepts water less

Complex Stratified Forests

• refers to ecosystems that exhibit a high degree of vertical layering of vegetation(stratification)

• Intercepts water more

Single Tree Plantation

• refers to the concept or initiative of individuals planting trees, often promoted by organizations

• Single tree intercepts water less

Interception Loss

Cycle:

1. Precipitation is caught by vegetation

2. Water evaporates from leaf surface(interception storage) to atmosphere

3. Water lost from surface system

How human activity affect interception

• Deforestation

• Farming

Deforestation

Less interception, more runoff. Therefore flashier response in rivers, increased river discharge and risk of flooding. Less evapotranspiration

Farming

leads to increased exposure to sunlight and less surface storage. Less evapotranspiration

Surface or Depression Storage

• volume of precipitation temporarily held in small, low-lying areas(depressions) on land surface

• will either infiltrate into the soil, evaporate, or become a runoff

Evaporation

liquid changes into a gaseous state

Factors Affecting Evaporation

• Vapor Pressure

• Temperature

• Wind

• Atmospheric Pressure

• Soluble Salts

• Heat Storage in Water Bodies

Vapor Pressure Formula

Dalton’s Law

EL = C(es - ea)

EL - rate of evaporation

C - coefficient of evaporation

es - saturation vapor pressure at water temperature

ea - actual vapor pressure in air

Temperature

• rate of evaporation increases as water temperature increases

• air temperature don’t have high correlation between evaporation rate with increasing temperature

Wind

• aids in removing evaporated water vapor from the zone of evaporation, creating greater scope for evaporation

• if wind velocity is large enough to remove all evaporated water, any increase to wind velocity doesn’t influence evaporation

• rate of evaporation increases with wind speed up to a critical speed beyond which any further increase of wind speed has no influence in evaporation rate

Atmospheric Pressure

decrease in barometric pressure (such as in high altitude) increases evaporation

Soluble Salts

• vapor pressure of solution is less than that pure water which causes reduction of rate in evaporation

• evaporation from seawater is about 2-3% less than freshwater

Heat Storage in Water Bodies

• deep water bodies have more storage than the shallow ones

• deep bodies of water may store more radiation energy in summer and release it in winter causing less evaporation in summer but more in winter

Estimation of Evaporation

• Evaporimeters

• Analytical Methods

• Empirical Equation

Evaporimeter

1. Class A Evaporation Pan

• used by US Weather Bureau

• aka Class A Land Pan

• made of unpainted galvanized iron sheet

• measurements are made by measuring the depth of water with a hook gauge in a stilling well

Evaporimeter

2. Colorado Sunken Pan

• square pan made of unpainted GL sheet, buried on ground

• its aerodynamic and radiation characteristics are similar to a lake

• difficult to detect leaks, expensive to install, extra care is needed to keep surrounding areas free from tall grass, dust, etc

Evaporimeter

3. USGS Floating Pan

• square pan of 900mm sides, 450mm deep

• supported by drum floats in the middle of raft with size 4.25m x 4.87m

• set afloat in a lake with a view to simulate the characteristics of large body of water

• water level in the pan is maintained at same level as in the lake

• diagonal baffles are provided in the pan to reduce surging in the pan due to wave action

• high cost of installation and maintenance, difficulty in measurements

Pan Coefficient

Lake evaporation = Cp x pan evaporation

Cp = pan coefficient

Water Budget Method

• simplest but least reliable

• involves writing the hydrological continuity equation for the lake and determining the evaporation from a knowledge or estimation of other variables

P + Vis + Vig + Vos + Vog + EL + S + TL

Energy-Budget Method

• law of conservation of energy

• energy for evaporation is determined by considering the incoming, outcoming, and stored energy in a water body over a known time interval

• energy in terms of calories per square mm per day

Mass-transfer Method

based on theories of turbulent mass transfer in boundary layer to calculate mass water vapor transfer from surface to surrounding atmosphere

Transpiration

• plants release water vapor through small pores(stomata) on their leaves and stems

• important part of hydrologic cycle, typical mechanism on how precipitated water in land brings back to atmosphere

Evapotranspiration (ET)

• combined evaporation and transpiration

• maximum if water supply to plant and soil is unlimited

• max amount of water that can be evaporated and transpired if there was unlimited supply of water is potential evotranspiration

Field Capacity

moisture content above which water will be drained by gravity

Wilting Point

moisture content below which plants cannot extract further water