Geoff Appleby, October 1, 2009
Chapter 6 Summary: Soil and the Hydrologic Cycle
6.1 The Global Hydrologic Cycle
-97% of water is found in the oceans, (long residence time compared to rivers/lakes).
-driven by solar energy, water absorbs about 1/3 of the energy that reaches the Earth.
-watershed = area of land drained by a single system of streams
6.2 Rate of Precipitation and Irrigation Water
-interception: water from precipitation intercepted by plant material, never reaching the soil (in some
forested areas up to 30 – 50 %).
-infiltration: water penetrating into the soil, if it exceeds soil capacity runoff may occur.
-soil storage: water retained by the soil, much of it absorbed by plants (affected by type of veg, soil
management, and soil properties).
6.3 The Soil-Plant-Atmosphere Continuum
-combined loss of water through transpiration and evaporation is evapotranspiration
-potential ET: how fast water vapor lost if soil water was at optimum level.
-water stress forces plants to close stomata which causes, (a) plant to stop growing due to insufficient
CO2, and (b) the plant to heat up due to the lack of evaporative cooling.
6.4 Control of Evapotranspiration
Control of Transpiration
-weeds can interfere with the growth of plants
-the use of herbicides also has advantages (less labor, energy, soil undisturbed), and disadvantages (high
cost, weeds evolving making them resistant)
-farming systems that leave sections fallow can help to conserve soil moisture.
Control of Surface Evaporation
-use of mulch, conservation tillage
Percolation and Leaching
-percolation losses are influenced by: rainfall and distribution, runoff, evaporation, character of soil,
nature of vegetation.
6.5 Liquid Loss of Water from the Soil
-two types: (i) percolation or drainage water, (ii) runoff water
6.6 Percolation and Groundwaters
-water table is the surface section of the saturated zone known as groundwater.
-unsaturated zone above the water table is known as the vadose zone
-capillary fringe: zone of wetting by capillary movement
6.7 Enhancing Soil Drainage
Reasons for Enhancing Soil Drainage
-engineering problems, plant production
-benefits of artificial drainage: a) increased bearing strength, b) less frost heaving, c) improved oxygen
supply, d) reduced levels of fungal disease, e) more rapid soil warming, f) less methane/nitrogen
g) removal of excess salts
-detrimental effects of artificial drainage: a) loss of wildlife habitat, b) reduction in nutrient assimilation
c) increased leaching of nitrates, d) accelerated loss of soil organic matter, e) increased frequency/severity
of flooding, f) greater cost of damages when flooding
Drainage Systems
-ditches and land smoothing to avoid puddles, underwater perforated pipes
6.8 Septic Tank Drain Fields
-water exiting septic tank is termed septic tank effluent
-the soil in the field has 3 jobs: keep the effluent out of sight, treat/purify the effluent, and conduct the
effluent into the groundwater
6.9 Irrigation Principles and Practices
-landscaping, food production: irrigation consumes 80% of worldwide water consumption
- drip or trickle irrigation is best established, most efficient