Question

2. A soil was found to have the following water retention characteristics: Water content at -10 kPa (field capacity) = 0.15 kg kg-1; water content at -1500 kPa (wilting point) = 0.06 kg kg-1. The bulk density was found to be 1250 kg m-3. Assume the density of water to be 1000 kg m-3. 2.1 Calculate the plant available water capacity (PAWC), expressing your results in depth units. [2] 2.2 The crop growing in this field has a rooting depth of 40 cm. Calculate the plant available water (PAW) for this crop? [1] 2.3 A farmer irrigated his soil to field capacity then allowed it to dry out so that the PAW was 65% of its original value before he reapplied irrigation. Calculate the volume of water (per hectare of land) that the farmer must apply to raise the soil water content from the depletion level back to field capacity for the rooting depth of the crop.[3

Answer 1

Given:

Water content at field capacity = 0.15 kg kg-1

Water content at wilting point = 0.06 kg kg-1

PAWC = Water content at field capacity - Water content at wilting point

PAWC = 0.15 kg kg-1 - 0.06 kg kg-1

PAWC = 0.09 kg kg-1

To convert kg kg-1 to depth units, we need to multiply by the bulk density of the soil, and divide by the density of water.

Bulk density of soil = 1250 kg m-3

Density of water = 1000 kg m-3

PAWC in depth units = (PAWC * Bulk density of soil) / Density of water

PAWC in depth units = (0.09 kg kg-1 * 1250 kg m-3) / 1000 kg m-3

PAWC in depth units = 0.1125 m

So, the Plant Available Water Capacity (PAWC) is 0.1125 m or 11.25 cm.

2.2 Plant Available Water (PAW) for the crop with rooting depth of 40 cm can be calculated as:

PAW = PAWC * Rooting depth

PAW = 0.1125 m * 0.40 m

PAW = 0.045 m or 4.5 cm

So, the Plant Available Water (PAW) for the crop is 0.045 m or 4.5 cm.

2.3 To raise the soil water content from the depletion level back to field capacity for the rooting depth of the crop, the farmer needs to apply water equivalent to the difference between PAW at depletion level and PAWC, per hectare of land.

Given:

PAW at depletion level = 65% of PAWC = 0.65 * 0.1125 m = 0.073125 m

PAWC = 0.1125 m

Volume of water to be applied = (PAWC - PAW at depletion level) * Area of land

Let's assume the area of land is 1 hectare, which is equivalent to 10,000 m^2.

Volume of water to be applied = (0.1125 m - 0.073125 m) * 10,000 m^2

Volume of water to be applied = 0.039375 m * 10,000 m^2

Volume of water to be applied = 393.75 m^3

So, the farmer must apply 393.75 m^3 of water per hectare of land to raise the soil water content from the depletion level back to field capacity for the rooting depth of the crop.