(a) Fig. 1.1 shows a cross-section of a field with sub-surface drainage pipes installed. Fig. 1.1 (i) State two features that should be considered when deci...

Labelled answer diagram:

(a)(i) Two features to consider when deciding the spacing between sub-surface drainage pipes:

1. Soil type and texture - clay soils have very small pore spaces and conduct water slowly, so pipes must be spaced closer together (often 5-10 m) to intercept water before it causes waterlogging. Sandy soils drain faster and allow wider spacing (15-25 m). [1]
2. Depth of the water table - if the natural water table is high, closer pipe spacing is needed to lower it to a depth that allows healthy root growth. [1]

(Also acceptable: local rainfall intensity, or the rooting depth of the intended crop.)

(a)(ii) Gravel placed around drainage pipes serves two functions:

1. Filtration - the gravel acts as a physical filter that prevents fine soil particles (especially clay and silt) from entering and blocking the pipe perforations or slots. Without this filter layer the pipes would clog over time. [1]
2. Improved water flow - gravel has large, interconnected pore spaces that conduct water much more freely than the surrounding soil. It creates a highly permeable zone around each pipe, drawing water from a wider area of the soil towards the pipe inlet. [1]

(b) The two drainage types differ in where they remove excess water from:

[2]

(c) Rice paddies are deliberately flooded for much of the growing season, but controlled drainage is still necessary at specific stages:

1. Harvest preparation - the field must be drained and allowed to firm up so that workers or machinery can enter without sinking. [1]
2. Crop health management - periodic draining between flooding cycles allows weed control, fertiliser application, and aeration of roots. Continuous unbroken flooding can promote root diseases and oxygen starvation at certain growth stages. [1]