Chapter 2

Catchment Hydrology: Land-Atmosphere Interactions

• focus on precipitation and evapotranspiration
• definition of precipitation, evaporation and transpiration
• what processes govern et? what happens in a deforested catchment area?
• earliest measurements of precipitation, attributed to Kautilya, an Indian

chancellor of the exchequer during the fourth century B.C. were used as a
basis for taxation (high precip => high agricultural production)
• historical example: reclaimation of the arid west: Colorado river, measurements of p-et through discharge of the river from 1896-1921 yielded 16.8 mill acre feet (1922-76: 13.9 million acre feet (excercise? using USGS data?)

2.2 Precipitation

• definition: deposition of liquid water droplets and ice particles that are formed in the atmosphere and grow to a sufficient size so that they are returned to the Earth's surface by gravitational settling. Solid and liquid. Dew and fog do not count as precipitation (can be 5-10% in Pacific Northwest.
• There are three primary steps in the generation of precipitable water in the atmosphere:
• (1) creation of saturated conditions in the atmosphere
• (2) condensation of water vapor into liquid water
• (3) growth of small droplets by collision and coalescence until they become large enough to precipitate
• explanantion of processes through the vapor pressure diagram
• case study: global precipitation patterns
• most precipitation comes from bordering oceans, but up to 40% can come from local ET.
• extremes in US: Kauai: 12,000 mm/y, Death Valley: 40mm/y
• Atacama deset is the driest place on Earth
• average annual precipitation onto the continents is a function of:
• (a) latitude (precipitation highest in latitudes of rising air-0° and 60° north and south-and lowest in latitudes of descending air- 30° and 90° north and south);
• (b) elevation (due to orographic cooling, precipitation usually increases with elevation);
• (c) distance from moisture sources (precipitation is usually lower at greater distances from the ocean);
• (d) position within the continental land mass;
• (e) prevailing wind direction;
• (f) relation to mountain ranges (windward sides typically cloudy and rainy, with leeward sides typically dry and sunny)
• (g) relative temperatures of land and bordering oceans
• excercise: look at global (Fig) and US (Fig) patterns in annual precipitation and relate them to the above trends
• world precipitation is extremely variable in time (examples?)

2.2.1 Point measurements

• Obviously precipitation data are extremely important in hydrology
• need to measure at many points and need to extrapolate
• point measurements performed by recording and non-recording gages
• tipping bucket rain gage
• snow depth measurements by telemetry, 500 remote sites in US
• precipitation typically measured as depth
• many stations all over the world (Fig)
• the record of hourly precipitation over time is called a hyetograph and shows that precip is organized into discrete storms (Figure 2.3, a station in North Carolina)

2.2.2 Spatial characteristics of precipitation and radar estimation

• for hydrological purposes, it is importnat to know te average precipitation over an area.

Moisture in the atmosphere

• water undergoes huge expansion during evaporation: 1g of water equals 1 ml volume in liquid form and 42 l as vapor (at 25^oC)
• water vapor pressure as a function of temperature (svp = saturation water vapor pressure) (Fig)
• absolute humidity (or water vapor mixing ratio): mass of vapor per unit volume of air, in g m^-3
• at 30^oC, air has a svp of 42.43 hPa (hPa = mbar) and can contain up to 30 g m^-3, at 0^oC svp is only 4.5 g m^-3
• relative humidity: actual water vapor pressure / svp in %; or: actual water vapor content / absolute humidity

formation of fog, clouds, mixing clouds, can be understood in the framework of the vapor pressure diagram (Fig)