Hydrology Chapter 2

EES 210

What are the conditions needed for rainfall to occur?

1. Humid air mass must be cooled to the dew point temperature

2. Nuclei must be present

3. Raindrops must be sufficiently large

What is vapor pressure?

It is how the moisture in the air, or water vapor, can be measured. The more water that evaporates into the air, the higher the vapor pressure.

What is saturation of vapor?

It is the maximum amount of moisture in air at a given temperature.

What is the equilibrium condition?

It is when equal numbers of water molecules are entering and leaving the surface of water

What is the dew point?

It is the temperature that air must cooled to reach saturation

What is saturation vapor pressure?

Vapor at this pressure is saturated. The dew point temperature is the same as the air temperature.

What is relative humidity?

It is the ratio of the current actual humidity to the highest possible humidity. RH = (e_a/e_s) * 100%

When temperatures increase, relative humidity _______

decreases, warm air can hold more water vapor.

How can air become saturated?

1. Evaporate more water into it

2. Decrease the temperature

What is the orographic effect?

It is the effect where rainfall amounts increase dramatically as you move farther up a mountain on the windward side. This is because the air cools down as it rises.

What is the rain shadow effect?

It is the effect where precipitation amounts drop dramatically on the leeward side of a mountain

What is the dry adiabatic lapse rate?

It is the rate at which an unsaturated air parcel changes temperature as it moves vertically through the atmosphere without exchanging heat with its surrounding environment

What is the saturated adiabatic lapse rate?

It is the rate at which a saturated air parcel cools as it rises vertically and expands in the atmosphere

What is NEXRAD?

It stands for Next Generation Radar and it measures precipitation by emitting a radar.

What is a hyetograph?

It is a graphical representation of the distribution of rainfall over time

What type of distribution is used for annual precipitation?

Normal

What is the arithmetic mean method?

It is a simple average. It takes the precipitation measured at several points and calculate the average. It is best for evenly spaced gauges over a fairly uniform area.

What are isohyetal lines?

They are contour lines of precipitation.

What is this equation?

It is the estimation of missing precipitation data. P1 is the missing precipitation value at station 1. P2….P_N are index stations, divided by the mean annual precipitation from station 1 and index stations.

What is this ?

An alternative method for estimating missing precipitation. P2-PN are index stations / D2-DN are the distances between index stations and station one.

What is interception?

It is the precipitation captured by plants before reaching the ground and is evaporated back to the atmosphere. It is highest during short, light storms. It is lowest during heavy, long storms

How much of precipitation is intercepted by the canopy?

8-35%

What is stem flow?

It is excess water on leaves that flows through stems and trunksW

What is throughfall?

It is precipitation that passes through the plant canopy

How much energy is needed to evaporate water?

600 cal of heat / gram of water

What are factors affecting evaporation?

heat energy, capacity to transport vapor away, water availability

What is water potential?

It is defined as the potential energy of water per unit of volume

How are sap flow measurements taken?

Two probes are inserted into the xylem. The upper is heated and the lower compares the temperature difference. It reaches its maximum when sap flow is close to zero, it decreases when water comes up.

What are factors affecting transpiration?

1. relative humidity

2. air movement

3. temperature

4. atmospheric pressure

5. water supply

6. light intensity

What is this equation?

It measures evapotranspiration.

What is this?

Lysimeter ET estimation

What is a lysimeter

It is a large container that holds soil and plants and measures evapotranspiration.