Lecture 6: Soil Water

Transcription

1 6-1 GEOG415 Lecture 6: Soil Water Infiltration Movement of water into soil. Importance? - flood prediction -erosion -agriculture - water resources Infiltration capacity Expressed in the same unit as rainfall intensity (mm hr -1 ). Significant overland flow occurs when infiltration capacity is exceeded by heavy rainfall. Dunne and Leopold (1978, Fig. 6-1)

2 6-2 Infiltration process Driving force - gravity - surface tension (capillary force) wicking Finer soils have stronger capillary force. Why? h Dunne and Leopold (1978, Fig. 6-2) Permeability of soil depends on pore sizes. Why? perimeter driving force flow resistance area Infiltration capacity decreases with time during a storm event. - reduction of capillary force - pore blockage by fines - soil swelling

3 6-3 Controls of infiltration Three processes are involved in infiltration: -entry -storage -transmission What control these processes? - pore sizes - macro features (cracks, root holes, etc.) - depth of the permeable soil - rainfall characteristics - antecedent soil moisture condition. -frozen soil infiltration rate (mm/hr) Infiltration test, Saskatoon grass cultivation time (min)

4 6-4 Effects of vegetative cover - protects soil surface from direct impacts of rain drops. - enhances soil structures and macro features -?? Effects of cultivation? Zero till an alternative to conventional cultivation. Infiltration measurement Single-ring infiltrometer - constant water level in the ring - overestimates infiltration. Why? - good for comparing the relative magnitudes of infiltration. Plot/watershed study The amount of overland flow during a storm is measured and compared against the total precipitation. This method is used to estimate spatially averaged infiltration capacity. Does this really measure infiltration capacity? See next page for an example

5 6-5 Dunne and Leopold (1978, Fig. 6-6) Estimation from soil/vegetation type A rough, semi-quantitative estimate of infiltration capacity can be made from soil type and the characteristics of the surface (e.g. vegetation). See DL, p.171.

6 6-6 Soil moisture storage Soil moisture is a minor component in terms of relative quantity (0.064 % of world s fresh water reserve), but it is a very important component of the hydrologic cycle. Why? Soil water is held by tension, which refers to the force that soil particles exerts to pull water molecules against gravity. Tension consists of capillary force and osmotic force. Capillary force is generally much higher. Exception? Unit of tension Dunne and Leopold (1978, Fig. 6-8) 1 atm = N m -2 = 10.3 m of water column Soil tension is measured by a tensiometer. vacuum gauge The soil absorb water through the porous cup. What happens to pressure in the air pocket? When will the flow stop? air pocket water We can estimate soil tension from the pressure in the air pocket. ceramic cup

7 6-7 Soil moisture measurement Gravimetric method - Weigh a known volume of sample. - Dry it in an oven, and weigh again. - Accurate, but non-repeatable. Neutron scattering method - Neutrons are emitted from a probe placed in a tube. - They are scattered and slowed down by hydrogen atoms. - The number of returning slow neutrons is related to moisture content by a calibration procedure. - Sampling volume : 30 cm radius. Time domain reflectometry (TDR) - Speed of electromagnetic (EM) wave m/s in air m/s in water Higher water content results in slower velocity. - Send EM waves into the soil and measure the velocity. - High spatial resolution (< 10 cm). - Usually limited to a small depth (1-2 m). - EM waves are easily lost in saline soils. TDR box counter neutron source EM waves Reflected at the end of wave guides

8 6-8 Soil-moisture characteristics The relationship between water content and soil tension. This is measurable in the field and laboratory. 100 Clay-loam soil, Saskatoon soil tension (m) field laboratory water content Field capacity refers to the soil-moisture condition after free drainage of soil. It is commonly represented by soil tension of 1 m. Why? Wilting point refers to the condition when all plant-accessible water is depleted. It is commonly represented by soil tension of 150 m.

9 6-9 Soil-moisture monitoring in a clay-loam soil under a wheat field near Saskatoon Average water content in top 60 cm Soil tension at 40 cm. water cont. tension Harvest in early Sep. 4 3 water content Heavy rain on Aug soil tension (m) 0.3 6/25 7/10 7/25 8/9 8/24 9/8 9/23 10/ Soil water content in a sandy loam at a depth of 0.3 m. Dunne and Leopold (1978, Fig. 6-10)

10 6-10 Porosity of most natural soils = Field capacity depends on the soil texture. Why? Dunne and Leopold (1978, Fig. 6-9) Note that the heavy textured soil may have a fair amount of water, but this water is not accessible to plants.

11 6-11 Movement of soil water Driving force: gravity and the gradient of soil tension. Water has a tendency to move to drier regions. Soil s ability to transmit water is called hydraulic conductivity. Conductivity increases with increasing moisture. Why? saturated soil drier soil rain 0 water content? Gravity-driven flow. Why? High gradient of tension wetting front

12 6-12 Water is slowly redistributed in the soil after the rainfall event. Gravity and tension gradient are still the driving force, but flow resistance is much higher in unsaturated soil. Why? What is the implication on evaporation and transpiration? Dunne and Leopold (1978, Fig. 6-14) Summer fallow in the prairies Dry land agriculture requires leaving the land cultivated but unseeded every 3-4 years. This practice is called summer fallow. Very little evaporation occurs on the summer-fallow field and soil moisture is conserved during the summer, which reduces the moisture deficit in the following year.