Soil-moisture-irrigation relationship

Soil acts as a reservoir for the water used by crops, and irrigation refills it. The relationship is explained by the soil-moisture constants.

 Moisture
 content
  Saturation  ---- gravity water drains ----
  FC          ---- upper limit (irrigate up to here)
                |  available moisture
  RAM limit   ---- (allowable depletion) ----
  PWP         ---- plant wilts; lower limit
  Hygroscopic ---- not available

• Saturation: all pores are filled. The excess (gravity) water drains in 1-3 days.
• Field capacity (FC): the moisture held after free drainage; the upper limit of useful soil moisture.
• Permanent wilting point (PWP): the moisture at which plants wilt permanently; the lower limit.
• Available moisture = FC - PWP. Readily available moisture (RAM) is the portion (usually 50-75%) that the crop can take without stress.

Link with irrigation

1. Irrigation is given when moisture falls to the RAM limit, not to PWP.
2. Net depth of water per irrigation = the depth needed to bring the root zone back to FC:

$$d = \frac{(FC - \theta)}{100}\times \frac{\gamma_d}{\gamma_w}\times D_r$$

where $\theta$ is the moisture content (%) at the time of irrigation and $D_r$ is the root zone depth. 3. Frequency (interval) of irrigation = $d/ET_c$. 4. Water applied beyond FC is lost as deep percolation and may cause waterlogging; under-irrigation causes stress and yield loss.

Thus the soil moisture characteristics decide how much water to apply and how often.