Introduction to the Global Hydrologic Cycle and Water Budget, Part 1

Tamlin Pavelsky, Associate Professor of Global HydrologyDepartment of Geological Sciences

University of North Carolina

San Diego, CA

Fairbanks, AK

Mean Annual Precip.: 26 cm

San Diego, CA

Mean Annual Precip.: 27 cm

Fairbanks, AK

If we want to understand changes in water resources, we need to examine the whole global hydrologic cycle.

Evaporation and Transpiration

Measuring Evapotranspiration

The typical way of measuring evaporation is via the Class A evaporation pan. The pan is filled with water to a specified line at the beginning of the observation day. At the end, it is refilled to the same line. The amount of water poured in represents the evaporation.

Metal screen keeps animals from drinking.

Measuring Evapotranspiration

The typical way of measuring evaporation is via the Class A evaporation pan. The pan is filled with water to a specified line at the beginning of the observation day. At the end, it is refilled to the same line. The amount of water poured in represents the evaporation.

Advantages: Low tech, inexpensive, accurate under most circumstances.

Disadvantages: Overflows during big rainfall events, doesn’t account for limited water supply in the actual environment.

Metal screen keeps animals from drinking.

Eddy Flux Tower

Evaporation can be calculated most successfully using a series of measurements made at different elevations above the land surface from a structure called an Eddy Flux Tower. However, these are expensive to construct and it isn’t feasible to build a large number of them.

Global Eddy Flux Tower Network (Fluxnet)

There are only about 500 permanent, reliable sites worldwide, and they aren’t evenly distributed.

Eight Different Model Estimates of Global Evapotranspiration

Jimenez et al. (2011), Journal of Geophysical Research

Precipitation

Standard Rain Gauge with Windscreens

Tipping Bucket Rain Gauge

Problems with Standard Precipitation Gauges

Undercatch of precipitation ranging from ~5% to more than 50% from sources such as:

Turbulence over the gauge opening due to wind* Evaporation of water within the gauge Water splashing into and out of the gauge

Snow can be very difficult to measure using a standard rain gauge because:

Wind-related turbulence is an even bigger problem Antifreeze has to be added to the gauge to melt incoming snow Snow can easily blow into the gauge from ground sources

*This is the biggest problem in terms of undercatch

Streamflow/Runoff

Measuring River Discharge and Runoff

Fundamental Parameters in River Discharge (Q, m3/s):

Depth(d), Velocity(v), Width(w)

Q=wdv

Example data from an Acoustic Doppler Current Profiler (ADCP), the best method we have for measuring discharge.

Measuring River Discharge

Fundamental Parameters in River Discharge (Q, m3/s):

Depth(d), Velocity(v), Width(w)

Q=wdv

w=aQb d=cQf v=kQm

These power-law relationships have been recognized for well over a century but were fully explored by famous hydrologists Luna Leopold and Thomas Maddock in the 1950s

Each of these variables can also be individually related to width:

Example of a Stream Rating Curve

In hydrology, discharge rating curves are statistical relationships between river discharge and one of the three dimensions of discharge variability: Velocity, Width or Depth.

Depth (or gauge height) is the variable most often used to create rating curves.

The USGS River Gauge

The USGS has been using the same basic design of river gauge to measure water depth (or stage) for decades.

Gauged Discharge from Throughout the United States

http://waterwatch.usgs.gov

Data Available in the Global Runoff Data Centre

Conclusion: Much of the world has no publicly available discharge data

Summary Points

If we want to understand the variability in Earth’s water resources, we need to examine all major components of the water cycle.

Over the last two centuries we have developed useful, standardized techniques for measuring precipitation, evaporation, and streamflow, as well as other parts of the water cycle.

These methods have significant limitations and are not available everywhere.

Analyzing Patterns in Global River Widths

Key Goals:

1. Develop a global river width database from satellite imagery.

2. Examine global patterns of river width and relationships between river width and river discharge.

3. Develop a global framework to estimate which rivers a new satellite mission, the Surface Water and Ocean Topography Mission, will observe.

4. Provide a series of workshops to provide training on remote sensing and the hydrologic cycle for North Carolina high school science teachers.

3-year NASA Grant funded by the New Investigators Program

Extra Slides

The Thorthwaite Method

Allows us to estimate potential ET with only mean monthly temperature

d=L/12, where L is the average day length for the month and location

T=the mean monthly air temperature in °C

a=(6.75x10-7)I3 - (7.71x10-5)I2 + (1.792x10-2)I + 0.49239

Ti is each mean monthly temperature for the year

Developed in 1948 by C.W. Thornthwaite, Professor of Climatology at Johns Hopkins (1899-1963), this method allows us to estimate potential evapotranspiration with nothing more than monthly air temperature.

The Penman Montieth Equation

D: How quickly the air saturates as evaporation occursRn-G: Net Radiation minus the ground heat fluxra: Density of the atmospherecp: Specific heat of watere*

a-ea: Difference between how much water the atmosphere can hold and how much it actually is holding.

ra: Resistivity of the atmosphere, which depends on wind speed and temperature.

g = the psychrometric constant, ~ 6.65x10-4 [hPa/K]rs: Resistivity of the land surface, which depends on how rough the

land surface is and how much water is available.

The Penman Montieth Equation

D: How quickly the air saturates as evaporation occursRn-G: Net Radiation minus the ground heat fluxra: Density of the atmospherecp: Specific heat of watere*

a-ea: Difference between how much water the atmosphere can hold and how much it actually is holding.

ra: Resistivity of the atmosphere, which depends on wind speed and temperature.

g = the psychrometric constant, ~ 6.65x10-4 [hPa/K]rs: Resistivity of the land surface, which depends on how rough the

land surface is and how much water is available.

How often do we know all of this?