CE 394K.2 Hydrology                       Homework 3: Soil Water Balance

Part A.

The spreadsheet soilwaterdata.xls attached to the course web site gives atmospheric flux and soil water balance data for the Freeman Ranch, as collected by Dr Marcy Litvak (thank you Marcy!!).   A portion of the spreadsheet is shown below.   What you have is latent heat flux (W/m2), and precipitation in mm for each 30 minute period for a week in August 2004, and volumetric soil water content for two soil layers (0 – 10cm, and 10 – 20cm).   All data are time-stamped at the beginning of the 30 minute period to which they apply.

 DateTime LH flux W m-2 soil_water_0_10cm soil_water_10_20cm rain_Tot mm 8/6/04 12:00 AM 5.281 0.0515269 0.0335943 0 8/6/04 12:30 AM 1.89 0.0513815 0.0331627 0 8/6/04 1:00 AM 4.234 0.0512473 0.0329639 0 8/6/04 1:30 AM 8.76 0.0510689 0.032785 0 8/6/04 2:00 AM 2.075 0.0509275 0.0327381 0

and more data go in here…..

 8/13/04 10:00 PM -13.508 0.103742 0.10171 0 8/13/04 10:30 PM -6.058 0.103153 0.102137 0 8/13/04 11:00 PM 4.745 0.102582 0.102677 0 8/13/04 11:30 PM 1.444 0.101935 0.103237 0

(1)  Prepare plots of the four measured variables over the sampling interval.   What can you say about the effect of the rainfall on August 6?   Don’t pay too much attention to the flip in latent heat flux when the rainfall occurs – Marcy says that is likely a data anomaly that results from the effect of rainfall on the sampling equipment.

(2)   Convert the latent head flux into evaporation rate in mm/day for each 30 minute period and make a revised estimate of the actual evaporation during the periods when rainfall occurs.

(3)   Construct a daily soil water balance for the site.  This should include for each day:

• Precipitation and evaporation in mm/day;
• Soil water storage in the two soil layers in mm of water;
• Infiltration rate in mm/day at the soil surface;
• Soil water flux rate in mm/day at 10cm depth and 20 cm depth;

In making this soil water balance, assume that there is no runoff from the site during the data period.   Note that you can compute one more storage value (at the end of the data period) that you can the flux variables (which apply over each day rather than to time instants between days).  Assume that downward soil water fluxes are positive in this computation, the same as we assume with infiltration.

Part B – do Problems 4.1.1, 4.3.2 in the course text.