Hydrology BC ENV 3025

Answers - Homework # 7

1) (8 points) A soil sample is characterized by a volume of 100ml and a weight of 192g. After drying the sample in an oven at 105 oC for a day, the weight was reduced to 182g. The soil was then completely saturated with water resulting in a weight of 212g. Finally, the sample was drained (removal of gravity water) and the weight was determined as 187g. Calculate the (a) porosity, (b) moisture content at the beginning, and (c) the field capacity of the soil in %. Determine also the (d) density of the solid phase (only the soil grains) in g/ml. The density of water is 1g/ml.

a) porosity = Vp (that is Vc + Vg) /V. So, 212g - 182g = 30 g; 30g / 1g/ml (density of water) / 100ml = 30 ml/100ml = .30  = 30 %.
b) moisture content at beginning. 192 g - 182 g = 10 g or 10 ml / 100 ml = .10 = 10 %.
c) field capacity is Vc/V and Vc is the water left in the soil that is held by capillary action. So, in this example, the water that remaind after the sample was drained is what
you need to use:187g - 182 g = 5 g or 5 ml/100ml = .05 x 100% = 5%.
d) The important thing to remember here is that the volume of the soil has not changed, it still takes up 100 ml, but not all of that volume is solid. So, 182 g/100 ml/(1-0.3) = 2.6 g/ml.

2) (8 points) Tensiometers are installed in a soil at 0.4 and 0.3 m above the water table in a uniform sandy soil with the moisture characteristics and hydraulic conductivity curves given in Figures 8.4 and 8.5, respectively. One set of tensiometer readings indicates that the capillary pressure head at the first of these tensiometers in -0.5m and at the second is -0.6m.

a) Make a little diagram of the set-up. Determine the hydraulic head at the two locations. Does the water flow upward or downward?

The flow is downward, because the hydraulic head decreases with depth. See graph.

b) Estimate the hydraulic conductivity in the soil and calculate the specific discharge using Darcy's law.

Moisture content from Fig 8.4 is about 0.06. Figures 8.4 and 8.5, can then be used to estimate hydraulic conductivity: ~10-7 m/s. The specific discharge then is: q = -K*(Dh/Dz) = 2 * 10-7 m/s.

3) (6 points) Unsaturated zone

a)   The infiltration rate is often decreasing during a precipitation event. What processes causing this behavior can you imagine?  (Identify 3 of the potential processes).

Initially the shallow layer of a dry soil takes up water very rapidly, there is lots of pore space to be filled. A wetting front forms and the infiltration rate is limited by how fast that front progresses (the hydraulic conductivity is dependent on the moisture content). Other potential reasons is that air gets trapped below the wetting front and resists the downward flow. If the unsaturated zone is very shallow, it could fill up completely and infiltration could stop. Swelling of minerals as a result of wetting could decrease the porosity as well and reducing infiltration with time. 

b) Does this have any implications for flood forecasting?

While the unsaturated zone can absorb considerable amounts of water in the initial phases of a storm, a reduced infiltration rate will result in a higher fraction of runoff and therefore intensify floods.