1. Introduction to the Global Hydrologic Cycle
and Water Budget, Part 1
Tamlin Pavelsky, Associate Professor of Global Hydrology
Department of Geological Sciences
University of North Carolina

2. Fairbanks, AK
San Diego, CA

3. Fairbanks, AK
Mean Annual Precip.: 26 cm
Mean Annual Precip.: 27 cm
San Diego, CA

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

5. Evaporation and Transpiration

6. 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.

7. 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.

8. 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.

9. Global Eddy Flux Tower Network (Fluxnet)
There are only about 500 permanent, reliable sites worldwide, and they aren’t evenly distributed.

10. Eight Different Model Estimates of Global Evapotranspiration
Jimenez et al. (2011), Journal of Geophysical Research

11. Precipitation

12. Tipping Bucket Rain Gauge
Standard Rain Gauge with Windscreens

13. 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

14. Streamflow/Runoff

15. 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.

16. 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
Each of these variables can also be individually related to width:
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

17. 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.

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

19. Gauged Discharge from Throughout the United States

20. Data Available in the Global Runoff Data Centre
Conclusion: Much of the world has no publicly available discharge data

21. 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.

22. Analyzing Patterns in Global River Widths
3-year NASA Grant funded by the New Investigators Program
Key Goals:
Develop a global river width database from satellite imagery.
Examine global patterns of river width and relationships between river width and river discharge.
Develop a global framework to estimate which rivers a new satellite mission, the Surface Water and Ocean Topography Mission, will observe.
Provide a series of workshops to provide training on remote sensing and the hydrologic cycle for North Carolina high school science teachers.

23. Extra Slides

24. 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
Ti is each mean monthly temperature for the year
Developed in 1948 by C.W. Thornthwaite, Professor of Climatology at Johns Hopkins ( ), this method allows us to estimate potential evapotranspiration with nothing more than monthly air temperature.

25. The Penman Montieth Equation
D: How quickly the air saturates as evaporation occurs
Rn-G: Net Radiation minus the ground heat flux
ra: Density of the atmosphere
cp: Specific heat of water
e*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.

26. The Penman Montieth Equation
How often do we know all of this?
D: How quickly the air saturates as evaporation occurs
Rn-G: Net Radiation minus the ground heat flux
ra: Density of the atmosphere
cp: Specific heat of water
e*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.