Soil Water Balance Reading: Applied Hydrology Sections 4.1 and 4.2 Topics Soil water properties Soil water flux Soil water balance Quotation for today (from Socrates) "There is only one good, that is knowledge; there is only one evil, that is ignorance." Contributed by Matt Harold.

Table 1. Size limits (diameter in millimeters) of soil separates in the USDA soil textural classification system. Soil Particle Sizes Name of soil separate Diameter limits (mm) Very coarse sand* 2.00 - 1.00 Coarse sand 1.00 - 0.50 Medium sand 0.50 - 0.25 Fine sand 0.25 - 0.10 Very fine sand 0.10 - 0.05 Silt 0.05 - 0.002 Clay less than 0.002

Soil Texture Triangle Source: USDA Soil Survey Manual Chapter 3

Soil Water Content Soil Water Content

Soil Water Flux, q q = Q/A

Energy fluxes at Freeman Ranch

Evaporation (mm/day) Average = 1.1 mm/day Average = 3.1 mm/day

Soil Water Content

Soil Water Balance at Freeman Ranch P E 0 cm Net Exchange 10 cm 20 cm

Soil Water Tension, y Measures the suction head of the soil water Like p/g in fluid mechanics but its always a suction (negative head) Three key variables in soil water movement Flux, q Water content, q Tension, y