Irrigation Methods and Soil Moisture Conservation
Irrigation Methods and Soil Moisture Conservation tests moisture constants, irrigation efficiencies, scheduling and the field practices that stretch every available millimetre of water.
Moisture constants
- Saturation: all pores filled. Field capacity (FC): water held after free drainage, about −0.1 to −0.33 bar. Permanent wilting point (PWP): about −15 bar, beyond the crop's reach.
- Available water = FC − PWP; hygroscopic water is unavailable.
- Infiltration is entry of water into soil; percolation is its downward movement.
Methods and scheduling
- Surface: flood, border, basin, furrow (efficiency roughly 40–60%). Sprinkler: 70–80%. Drip: 85–95%, best for wide rows, orchards and water-scarce areas.
- Schedule by stage of crop, critical stages (e.g., crown-root initiation in wheat, flowering in most crops) and the IW/CPE ratio.
- Conservation: mulching, dust mulch, ridges and furrows, conservation furrows, contour cultivation, straw mulch and reduced tillage.
Worked example
FC 26% and PWP 12% (gravimetric), bulk density 1.35 g/cm³, root zone 60 cm. Available water = (0.26 − 0.12) × 1.35 × 600 mm = 0.14 × 1.35 × 600 = 113.4 mm. Reasoning: the moisture difference is multiplied by bulk density to convert it to volumetric, then by depth to get a depth of water.
Traps
Using gravimetric moisture directly as a depth; reversing FC and PWP; claiming flood irrigation is the most efficient method; forgetting that drip wets only part of the root zone when computing the wetted area.