Gully on the move: ranking erosion forms in a 2.4 ha maize field
A farmer near Jhansi has grown continuous maize for six years on a 2.4 ha field of 3.5% slope, ploughing up and down the slope with a disc harrow. Annual rainfall is 850 mm, of which 78% falls from June to September, and the slope length is 180 m. After this monsoon he reports three things: a pavement of white stones and pebbles on the upper knoll where the soil used to be 20 cm deeper; a band of fresh soil 15–20 cm thick burying the lower boundary fence; and a channel near the field outlet that is now 60 cm deep, 1.8 m wide and 40 m long, with its headcut advancing about 1.5 m a year towards the village path. A 25 m long rill 10–12 cm deep appears each July and is erased by the next harrow pass. Erosion pins set in June show 14 mm of soil lost by October on the mid-slope (bulk density 1.3 g/cm3). Neighbours say rainfall has not changed. The farmer blames the seed variety and wants to re-level the field with a tractor-drawn leveller.
1. Convert the pin reading into an annual soil loss figure, compare it with a tolerable limit, and rank the erosion forms in the field by the urgency of treatment, giving your reason for each rank.
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Arithmetic: 14 mm over 1 ha = 0.014 m × 10,000 m2 = 140 m3/ha; at bulk density 1.3 t/m3 that is 182 t/ha in one season.
Comparison: tolerable soil loss for a medium-textured soil is commonly taken as about 5 t/ha/yr (roughly 2.5–12.5 t/ha depending on depth). The field is losing about 36 times the tolerable rate, while topsoil forms at roughly 1 t/ha/yr - the field is consuming more than a century of soil formation each monsoon.
Ranking:
- Gully - first. It is irreversible by cultivation, it is a direct hazard (headcut 1.5 m/yr; at that rate a path 20 m away is undermined in about 13 years), and it delivers sediment off-site. Its cross-section (60 cm deep, 1.8 m wide, near-vertical headwall) marks an active, youthful stage rather than a stabilised one.
- Sheet erosion on the knoll - second. The 182 t/ha figure and the stone pavement are the same process: the fines have gone and a lag gravel remains where 20 cm of topsoil used to be. This is the productivity loss and it cannot be recovered within a farmer's lifetime.
- Rill - third, but not negligible. It is significant erosion that is masked because each harrow pass erases the evidence. Treat it through surface cover and contour tillage, because it is the precursor of the gully.
Re-levelling would bury topsoil, expose subsoil and, by filling the gully head, simply shift the problem downslope. The seed variety is irrelevant; the fence deposit proves the field is exporting soil to a neighbour.
2. Design the treatment sequence from ridge to outlet, with at least one computed spacing, and explain why check dams alone would fail.
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Spacing: vertical interval = 0.3 × slope % + 0.3 = 0.3 × 3.5 + 0.3 = 1.35 m; horizontal interval = 1.35/0.035 = 38.6 m. Over a 180 m slope length that is 180/38.6 = 4.7, so lay out four bunds at about 36–38 m and use the last interval as a grassed waterway.
Sequence - treat the catchment before the gully:
- Agronomic first: contour sowing of maize, reduced tillage, residue mulch and a cover crop; vegetative filter strips of vetiver or khus on the contour every 20–30 m. This is the cheapest and fastest reduction in velocity.
- Mechanical: trench-cum-bund on the computed spacing (top width 0.45–0.50 m, height 0.5–0.6 m, side slopes 1:1, with a 0.3 m trench upslope supplying the fill). Keep the bunds slightly off-level towards a protected outlet.
- Waterway: a wide, shallow grassed waterway (Cenchrus or khus) at the outlet with stone pitching at the drop; never plant trees in it.
- Gully: reshape the headwall to a 1:1 or flatter batter and protect it with a masonry or gabion drop; place earth or brushwood check dams up the bed. With an effective check height of 0.5 m and a bed slope of 3%, spacing = 0.5/0.03 ≈ 17 m, i.e. two to three structures over 40 m; stabilise each with khus slips and fence against grazing.
Why check dams alone fail: they store sediment in the channel but do not stop the water or sediment arriving from 2.4 ha of eroding sheet and rill flow. The first storm that exceeds the check-dam storage either overtops or bypasses the structures, the headcut keeps retreating, and the dams silt up within a season or two. Gully treatment works only after the contributing catchment is treated.
3. Propose a monitoring plan for two seasons - what you would measure, how often, and what thresholds would tell the farmer the works are working.
Read the worked answer
The farmer needs evidence, not opinion, so build the plan around repeatable, low-cost measurements with pre-agreed thresholds.
Measurements:
- Erosion pins: reinstall at 10 m spacing on the mid-slope, three rows of six, read with a steel rule quarterly; average loss in mm converts to t/ha using bulk density 1.3.
- Gully head: drive a steel stake at the headcut and three permanent pegs along the rim; measure headcut advance and cross-section width after each monsoon.
- Outflow: a 2 m × 10 m runoff plot with a collection tank at the lower boundary, or a painted staff gauge in a small trapezoidal weir at the waterway outlet, read after every runoff-producing storm; record rainfall in a simple gauge beside the field.
- Deposition: mark a benchmark at the boundary fence to see whether the 15–20 cm deposition band stops growing.
- Visual check: count rills deeper than 5 cm after each monsoon, and photograph fixed points each month.
Thresholds for success: pin loss below 3.8 mm/season (about 5 t/ha), no headcut advance, no rills deeper than 5 cm surviving the monsoon, and no fresh deposition at the fence.
Interpretation logic: if pins improve but the headcut keeps moving, the water is arriving from above the field boundary and the catchment above needs treatment. If pins worsen despite bunds, check for breaching and for flow concentrated along the bund ends. Present the two seasons as a simple before-and-after table so the farmer can compare with the 182 t/ha baseline.