Introduction

Irrigation methods are grouped as surface (water flows over the soil), sub-surface (water reaches roots from below) and pressurised methods (sprinkler, drip).

Surface irrigation

• Wild (free) flooding: water is let loose over an unprepared, roughly level field and spreads under gravity. Cheap, but uneven and wasteful.
• Border strip: the field is divided into long, narrow, levelled strips separated by low bunds; water enters at the upper end and advances as a sheet.
• Check basin: the field is divided into small level plots enclosed by bunds; each plot is flooded to a required depth and left to infiltrate. Used for rice.
• Basin (ring basin): a circular basin around each tree, fed by a small channel. Used in orchards.
• Furrow: small parallel channels between crop rows carry water; the soil is wetted by seepage from the sides and bottom.

Suitability

Surface methods suit large flat areas (Terai), plentiful water and low-cost labour. They need land levelling; efficiency is 40-60%.

Sub-surface irrigation

• Natural sub-irrigation: the water table is kept high by seepage from canals or by controlled drainage, so that moisture reaches the root zone by capillary rise (low-lying fertile land, valley bottoms).
• Artificial sub-irrigation: water is supplied through open ditches or buried perforated/porous pipes (tile drains) laid below the root zone; the water table is raised to the required level.

 ground ~~~~~~~~~~~~~~~~~~~~~~~~~~~
        root zone  ^  ^  ^ capillary rise
 raised w.t. ------------------------
 perforated pipe  o==o==o==o==o

Suitability: permeable soil with an impervious layer below, flat land, a good natural drainage outlet, non-saline water. It saves water (no surface evaporation) and does not hinder farm operations, but it is costly and can cause salinity if drainage is poor.

Pressurised methods (for comparison)

Sprinkler suits sandy or undulating land and scarce water. Drip suits orchards and high-value vegetables on steep or sandy land where water is very scarce.

Methods of surface irrigation

• Wild (free) flooding
• Border strip
• Check basin
• Ring basin (orchards)
• Furrow

Methods of sub-surface irrigation

• Natural sub-irrigation (high water table, canal seepage)
• Artificial sub-irrigation (open ditches or perforated pipes below the root zone)

Furrow irrigation

Water runs in small channels (furrows) between rows of the crop and soaks sideways and downwards. Only part of the surface (about 1/3 to 1/2) is wetted.

  ridge  furrow  ridge  furrow  ridge
  /\_____/\_____/\_____/\
   crop   ~~water~~  crop
  head ditch ======================>

Types: straight, contour (curved) furrows, corrugations (small), and level or graded furrows.

Design points: furrow length 60-300 m (shorter for sandy soil), spacing 0.5-1.0 m according to crop and soil, slope 0.2-3% (usually 0.5%), stream size to reach the end without erosion.

Suitability

• Row crops: maize, potato, sugarcane, cotton, vegetables.
• Medium to heavy soils; sandy soils need short furrows.
• Gentle slopes; not on very flat clay (waterlogging) or steep land without contouring.

Advantages: water does not touch the plant stem, less crusting, good aeration, efficiency 50-65%. Disadvantages: needs levelling and labour; some deep percolation at the head and runoff at the tail.

Necessity of irrigation in Nepal

1. Uneven rainfall: about 80% of annual rain falls in June-September; the remaining months are dry, so winter and spring crops need irrigation.
2. Agriculture-based economy: agriculture supports about 60% of the population and contributes about a quarter of GDP; food security depends on higher yield.
3. Higher yield and cropping intensity: irrigated land gives 1.5 to 2 times the yield of rainfed land and allows 2-3 crops a year.
4. Rising population: limited cultivable land (about 2.6 million ha) must feed more people, so productivity must rise.
5. Use of improved inputs: HYV seeds and fertilizer give full benefit only with assured water.
6. Rural development: employment, income, reduced out-migration; also hydropower/multipurpose benefits and groundwater recharge.
7. Uncertain monsoon and climate change: irrigation protects against delay, breaks and droughts.

Problems and challenges

• Difficult topography: steep, fragile hills; canals pass through landslide-prone areas; high construction and maintenance cost.
• Flood and sediment: monsoon floods damage headworks and canals; heavy sediment silts canals; river bed shifts.
• Low reliability and efficiency: poor O&M, seepage and leakage; many systems get water only in the monsoon (year-round irrigation is far less than the irrigated area).
• Weak institutions: poor water user association (WUA) capacity, low service fee collection, limited technical staff, weak coordination among agencies.
• Funding: large capital needs; delays and cost overrun; dependence on donors.
• Water conflicts and competition among irrigation, drinking water, hydropower and industry; weak water rights.
• Groundwater in the Terai is under-used or overused locally; energy for pumping is costly.
• Climate change causes glacier retreat, dry spells and extreme rain.
• Land fragmentation and small holdings make on-farm distribution difficult.
• Environmental and social issues: land acquisition, resettlement, waterlogging and salinity.

Remedies include rehabilitation, conjunctive use of surface and ground water, micro-irrigation in hills, strong WUAs and joint management.

Operation and maintenance (O&M) means running the system to deliver water at the right time, place and quantity, and keeping the physical structures in good working condition. Poor O&M is the main reason many projects fail to give planned benefits.

Operation

Gate operation at headworks and regulators, flow control, water distribution by schedule (rotation, warabandi or demand), recording of discharge, and coordination with farmers.

Types of maintenance

1. Routine (annual) maintenance: done every year, usually in the closure period: desilting canals, removing weeds and vegetation, repairing banks, greasing and painting gates, cleaning drains.
2. Periodic maintenance: done after a few years: repair of lining, structures, bed levelling, replacement of gates and worn parts, re-sectioning of canals.
3. Emergency maintenance: immediate repair of breaches, damage by flood or landslide, blocked structures during the irrigation season.
4. Preventive maintenance: planned inspection and repair before failure occurs.
5. Rehabilitation/special repair: major reconstruction when the system has deteriorated.

Requirements

• Annual O&M plan and budget; trained staff and tools.
• Water user association participation and collection of irrigation service fee.
• Records of discharge, silt and damage; canal closure period for work.

Good O&M raises efficiency, extends life and gives equitable water distribution.

Nepal has about 2.64 million ha of cultivable land, of which roughly 1.76 million ha (mostly Terai) is considered irrigable by surface and ground water. These figures are approximate and are updated in national plans.

Present status

• About 1.3-1.4 million ha (roughly half of the cultivated area) has some irrigation facility; however, only about 0.5-0.7 million ha gets year-round irrigation.
• Surface irrigation (canals from rivers) dominates; groundwater (shallow tube wells, deep tube wells, STW) irrigates a large area in the Terai.
• Farmer-managed irrigation systems (FMIS) such as kulo in the hills cover a large share of the area; agency-managed systems (AMIS) cover large Terai schemes.
• Main schemes: Sunsari-Morang, Kankai, Narayani (Gandak), Chandra Canal, Rani-Jamara-Kulariya, Mahakali, Banganga, Babai and Bheri-Babai diversion (inter-basin), Sikta.
• Hills have small systems and are developing lift and micro-irrigation (drip, sprinkler).

Institutions and policy

• Department of Water Resources and Irrigation (DWRI) under the Ministry of Energy, Water Resources and Irrigation.
• Irrigation Policy 2060 BS, Water Resources Strategy 2002 and National Water Plan 2005, Irrigation Master Plan 2019 (targets year-round irrigation for all irrigable land).
• Management transfer to WUAs and joint management are being promoted.

Problems

Low reliability, silt, flood damage, poor O&M, low cropping intensity (about 150-180%) and limited year-round irrigation. The target is to extend year-round irrigation, use conjunctive water, and upgrade old systems.

Gross Command Area (GCA)

The total area enclosed within the boundary of a canal command, including cultivable land, roads, villages, ponds, forests and barren land. It is the area that could be irrigated by the canal by gravity.

Culturable Command Area (CCA)

The part of GCA that is fit for cultivation. CCA = GCA - unculturable area (settlements, rock, water bodies, roads). CCA is the area used for design.

Net Culturable (Cropped) Area (NCA)

The area actually cropped or irrigated in a season. It excludes land kept fallow and is less than or equal to CCA. For a single season, irrigated area is the part of CCA that actually receives water.

$$GCA > CCA > NCA$$

Often CCA is about 75-85% of GCA.

Cropping intensity

The percentage of the net sown area that is cropped in a year, counting each crop (a field with two crops in a year has 200%).

$$\text{Cropping intensity}=\frac{\text{Gross cropped area}}{\text{Net sown area}}\times 100$$

Irrigation intensity

Percentage of CCA irrigated in a season (e.g. 60% of CCA in kharif).

Cropping pattern

The yearly sequence and spatial arrangement of crops on a farm or in the command area, e.g. rice-wheat-fallow, rice-wheat-maize, or rice-mustard-maize. It depends on climate, soil, water, market and farmer's preference.

Sprinkler irrigation

Water is sprayed through nozzles under pressure like rainfall.

Advantages

• Suitable for undulating and steep land; no land levelling needed.
• Suitable for sandy and light soils with high infiltration.
• Saves water, efficiency 65-80%; uniform and controlled application.
• No runoff or deep percolation if designed well; saves land otherwise used for channels.
• Fertilizer and pesticide can be applied through the water; protects crops from frost and heat.
• Low labour.

Disadvantages

• High initial and energy costs (pump, pressure).
• Wind distorts the spray pattern, causing uneven application.
• Evaporation losses in hot dry weather.
• Not suitable for heavy clay soils with low infiltration or for rice.
• Wetting leaves can spread disease and causes scorch if water is saline.
• Needs skilled operation and clean water.

Drip (trickle) irrigation

Water is delivered drop by drop near the root through emitters and tubes.

Advantages

• Highest efficiency (85-95%); the least water for scarce areas.
• Wets only the root zone, so less weed growth and less evaporation.
• Excellent for orchards, vegetables and sloping land; works with saline water better.
• Fertigation is easy; high yield and quality; low labour.

Disadvantages

• High initial cost.
• Emitters clog (needs filters).
• Not suitable for closely spaced field crops.
• Tubes may be damaged by rodents, sun and farm operations.
• Salt may collect at the wetted edge; limited root spread.
• Needs training and maintenance.

Planning of an irrigation project is done step by step so that only viable projects proceed to costly design and construction.

1. Identification (reconnaissance) stage: potential sites are identified from maps, field visits and existing data. Water source, command area, rough cost and benefits are assessed. Output: a list of possible projects.
2. Pre-feasibility stage: a quick study of technical, economic, social and environmental aspects. Alternatives are compared, rough designs and cost estimates are prepared, and the project is screened.
3. Feasibility stage: detailed surveys (topographic, hydrological, geological, soil, agro-economic), preliminary design, quantity and cost estimate, benefit-cost analysis (B/C ratio, IRR, NPV), environmental and social impact assessment, and institutional/financial plans. Decides whether to proceed.
4. Detailed design and tender stage: final design of headworks, canals and structures, drawings, specifications, bill of quantities, tender documents.
5. Implementation (construction) and operation stage: construction, commissioning, O&M and later monitoring and evaluation.

Planning is also viewed at levels: national/master plan, river-basin (regional) plan, and project plan.

Cropping intensity

The ratio of the gross cropped area in a year to the net sown area, expressed as a percentage.

$$\text{Cropping intensity}=\frac{\text{Gross cropped area}}{\text{Net sown area}}\times 100$$

Example: 100 ha net sown, with 100 ha rice and 80 ha wheat, gives $\frac{180}{100}\times 100 = 180\%$.

Irrigation intensity

The percentage of the culturable command area (CCA) that is irrigated in a given season or year by the project.

$$\text{Irrigation intensity}=\frac{\text{Area irrigated}}{\text{CCA}}\times 100$$

Example: wheat irrigated on 3000 ha out of a CCA of 5000 ha gives 60%. Irrigation intensity may be given crop-wise (e.g. 30% wheat, 15% rice) and the sum over a year may exceed 100%.

Drip irrigation

Water is applied slowly, drop by drop, at or near the root zone through emitters on small tubes at low pressure (about 1-2 bar).

 Source -> Pump -> Filter -> Main line
                        |      |
                     Sub-main  Fertilizer tank
                        |
   ---o---o---o---o---o--- lateral (emitters)
   ---o---o---o---o---o---
        plant rows

Parts: pump, filter (screen/sand), fertilizer injector, main and sub-main pipes, laterals, emitters (drippers), valves, pressure regulator. Use: orchards, vegetables, greenhouse, steep and sandy land. Efficiency 85-95%.

Sprinkler irrigation

Water is pumped through a pipe network to nozzles, which spray it over the crop like rain.

 Source -> Pump -> Main pipe -> Lateral pipe
                                 |  riser
                      \ | /      \ | /
                       sprinkler   sprinkler
   ~~~~~~~~~~~~~ spray over field ~~~~~~~~~~~~~

Types: portable, semi-portable, permanent, rotating-head, perforated pipe, and mobile (raingun, centre pivot). Use: undulating, sandy and light soils, cereals, fodder, vegetables, and where water is scarce. Efficiency 65-80%.

Levels of planning

1. National (master) planning: long-term national or river-basin plan sets policy, priorities, water allocation among sectors and a pipeline of projects (e.g. Irrigation Master Plan).
2. Regional/basin planning: water resources of a river basin are studied; potential projects and inter-basin transfers are identified and ranked.
3. Project planning: for a chosen project, the stages are identification, pre-feasibility, feasibility, detailed design and implementation. Technical, economic, social and environmental viability are checked.
4. Operational (scheme) planning: seasonal cropping and water-delivery plans and management arrangements for an existing system.

Types of maintenance

• Routine (annual) maintenance: desilting, weed removal, bank repair, gate lubrication; done every year during canal closure.
• Periodic maintenance: repairs done after some years, e.g. relining, replacement of gates, re-sectioning.
• Emergency maintenance: urgent repairs of breaches, landslides, flood damage during operation.
• Preventive maintenance: inspection and minor work to avoid failure.
• Rehabilitation/modernisation: major repair or upgrading of an aged system.

Maintenance needs a budget from irrigation service fee and government, trained staff and active farmer participation.

Institutional aspects are the organisations, rules and people that plan, operate and maintain an irrigation system and share responsibility between government and farmers.

Main elements

1. Government agencies: Ministry of Energy, Water Resources and Irrigation and the Department of Water Resources and Irrigation (DWRI) with its regional/divisional offices plan, build and support the systems.
2. Water Users' Association (WUA): an organisation of farmers registered under the Irrigation Regulation (Water Resources Act 2049). It distributes water, collects irrigation service fee (ISF), maintains canals and resolves conflicts.
3. Management modes:
   • Agency management (government runs the system),
   • Joint management (shared duties, e.g. main canal by agency, branches by WUA),
   • Participatory irrigation management (PIM),
   • Irrigation management transfer (IMT) to WUAs,
   • Farmer-managed irrigation systems (FMIS), traditional in hills.
4. Legal framework: Water Resources Act 2049, Irrigation Policy 2060, regulations that define water rights and fees.
5. Support services: agricultural extension, credit, training, monitoring and evaluation.

Importance

Strong institutions give equitable distribution, sustainable O&M, higher fee collection, fewer conflicts and better crop yield.

Irrigation

Irrigation is the artificial application of water to land to supply the moisture that crops need for growth when natural rainfall is insufficient.

Disadvantages of irrigation

• Waterlogging and rise of water table from over-irrigation and seepage.
• Salinity and alkalinity of soil from salts left after evaporation.
• Mosquito breeding, so malaria and other water-borne disease may increase.
• High cost of construction and maintenance; possible displacement.
• Damp climate and cooler weather around large canals.
• Possible conflicts over water and loss of natural flow downstream.

Scope in Nepal

• Cultivable land is about 2.6 million ha, and a large part (especially in the Terai) can be irrigated, but only about half has irrigation, so expansion has wide scope.
• Rainfall is concentrated in the monsoon, so winter and spring irrigation is needed.
• Large perennial rivers (Koshi, Gandaki, Karnali, Mahakali) give abundant water for canals and groundwater in the Terai is plentiful.
• Hills have scope for small, lift, drip and sprinkler irrigation.
• It raises food production, farmers' income and supports multipurpose development.

Command area

The area that can be irrigated by gravity from a canal or project. Gross command area (GCA) is the total area; culturable command area (CCA) is the part fit for cultivation.

Cropping pattern

The yearly sequence and arrangement of crops grown in an area (e.g. rice-wheat-fallow).

Crop (cropping) intensity

The percentage ratio of gross cropped area to net sown area in a year.

Crop rotation

Growing different crops one after another on the same land in a planned order, e.g. rice - wheat - pulses.

Necessity of crop rotation

1. Maintains and improves soil fertility: legumes fix nitrogen, deep-rooted and shallow-rooted crops use different soil layers.
2. Controls pests, diseases and weeds, since their life cycles are broken.
3. Prevents exhaustion of the same nutrients.
4. Improves soil structure and organic matter.
5. Reduces waterlogging and salinity and spreads the water demand over the year.
6. Gives better use of labour and machinery, risk spreading and higher income.

The values below are typical for the mid-hills of Nepal (approximate; they vary with altitude, variety and soil, and textbook tables may differ slightly).

Notes

• Paddy needs standing water and has the highest delta.
• Wheat, a winter crop, is grown mostly with rainfall and a few irrigations.
• Delta is the total depth of water a crop needs over its base period, including rainfall contribution if not subtracted.
• Base periods are about 120-150 days for paddy, 100-120 days for maize and 120-140 days for wheat.

The irrigation method must fit the site, crop, water and farmers. The principal criteria are:

1. Topography: slope and land shape. Flat land suits basin, border and check methods; moderate slope suits furrow; steep or undulating land suits sprinkler or drip.
2. Soil: infiltration rate and water-holding capacity. Sandy soil suits sprinkler or drip; clay soil suits basin or border with low intake.
3. Crop: rice suits basin/check; row crops suit furrow; orchards suit basin, drip; close-growing crops suit border, sprinkler.
4. Water availability and quality: scarce water needs high-efficiency methods (drip, sprinkler); saline water favours drip or flooding with leaching.
5. Climate: wind and evaporation affect sprinklers; cold areas have frost protection needs.
6. Economic factors: capital cost, energy, cost of labour, land levelling, and market value of the crop.
7. Labour, skill and tradition: availability of skilled workers, farmers' acceptance.
8. Field size and shape: large regular fields suit mechanised surface methods and centre pivot.
9. Drainage and environment: drainage condition, salinity and erosion risk.
10. Water delivery system: whether water is available by pressure, gravity, or lift.

Sub-surface irrigation supplies water to the root zone from below the ground surface. The water table is raised or water is released below the surface so that it reaches the roots by capillary action.

Types

1. Natural sub-irrigation: the water table is naturally high, or seepage from canals or rivers keeps the root zone moist (e.g. low-lying river-bank lands).
2. Artificial sub-irrigation: water is fed through open ditches or buried perforated/porous pipes below the root zone; the water table is controlled by a gate or pump.

  ground ----------------------------
  root zone
  ~~~~~~~ raised water table ~~~~~~~~
  o=o=o perforated pipe
  ===== impervious layer ===========

Conditions for success

• Permeable surface soil with an impervious layer 2-3 m below.
• Flat land and a good drainage outlet.
• Low-salinity water.

Merits

Little evaporation loss, no interruption of farm operations, no erosion, very efficient use of water.

Demerits

Costly installation, risk of salt build-up in the root zone, waterlogging if drainage is poor, and difficulty in maintenance.

Surface irrigation

Advantages: simple, cheap, uses gravity (no energy), suits most crops and soils, easy to operate and learn. Disadvantages: low efficiency, runoff and percolation losses, levelling is needed, waterlogging and erosion on slopes, labour-intensive, weeds grow in wet channels.

Sub-surface irrigation

Advantages: saves water, no surface evaporation, no hindrance to cultivation, no weeds on the surface, good aeration and uniform moisture. Disadvantages: high cost, limited to special soil and topography, salt accumulation, pipes clog or get damaged by roots, control and checking of water distribution is difficult.

Surface irrigation methods

• Wild flooding: uncontrolled spreading of water.
• Border strip: levelled long strips between bunds.
• Check basin: small level plots with bunds (rice).
• Ring basin: circles around trees.
• Furrow: channels between crop rows.

Suitability of sprinkler irrigation

• Undulating and steep land (no levelling needed).
• Sandy and light soils with high infiltration.
• Where water is scarce and uniform light application is required.
• Close-growing crops: cereals, fodder, vegetables, tea; also for frost protection.
• Not suitable for heavy clay, windy sites, very saline water or rice.

Suitability of drip irrigation

• Orchards, plantations, vegetables, flowers and greenhouse crops (wide-spaced, high-value).
• Water-scarce areas and saline water, hills and sandy land.
• Where fertigation is wanted.
• Not suitable where water has too much silt or iron (clogging), or for closely sown field crops (cost).

In the hills of Nepal, land is steep, terraces are small, water is scarce and springs are far, so surface canals are costly and inefficient. Drip and sprinkler methods match these conditions.

Reasons

1. Steep and undulating land: no levelling needed; water follows pipes, not the slope, so there is little erosion.
2. Scarce water: efficiency of 65-80% (sprinkler) and 85-95% (drip) allows the small spring or stream flow to irrigate a larger area.
3. Gravity pressure: the difference in elevation between the source and the field provides the required head, so a pump and energy are not needed (gravity-fed drip works at 1-2 bar).
4. Small, scattered terraces: pipes can be laid to any plot; no long canals.
5. Pipes avoid seepage and landslide damage that is common in hill canals.
6. High-value crops: vegetables, fruit and off-season crops (tomato, cauliflower, citrus, tea) fit the market and give returns.
7. Light, shallow soils: frequent small doses suit the low water holding capacity.
8. Low labour requirement in areas with male out-migration; fertigation is possible.

Suitability

• Drip: orchards (citrus, apple), vegetables, polyhouse, with clean water.
• Sprinkler: cereals, fodder, tea and coffee on sloping land with a water head.
• Need clean filtered water, capital subsidy, farmer training and local service.

Situations for recommending sprinkler irrigation

1. Undulating or steep land where levelling is costly.
2. Light sandy soils with high infiltration, or very shallow soils.
3. Water scarcity and a need for uniform, controlled application.
4. Close-growing crops (cereals, pulses, fodder, vegetables, tea).
5. Frost protection, cooling or germination of seeds.
6. Where labour is costly or scarce and land is expensive (no channels).
7. Where pressure from gravity or an electric/diesel pump is available.

Advantages

• No land levelling; saves land used by channels.
• High efficiency (65-80%); no runoff or deep percolation if designed well.
• Even distribution; precise control of depth; can apply fertilizer and pesticides through it.
• Protects against frost; low labour.

Disadvantages

• High capital and energy cost.
• Wind spoils uniformity; evaporation in hot weather.
• Leaf wetting spreads disease and causes scorch with saline water.
• Not suitable for heavy clay soils or rice.
• Needs skilled operation, clean water and regular maintenance of nozzles.

Sprinkler irrigation, although efficient, has several practical problems.

1. Wind: wind drifts the spray, gives uneven distribution and reduces coverage; operation is difficult in strong wind.
2. High cost: pumps, pipes, sprinklers and energy raise initial and running costs; unaffordable for small farmers in Nepal.
3. Energy and pressure: needs adequate pressure; where electricity is poor or diesel is costly, running is difficult.
4. Evaporation loss from droplets in hot and dry weather.
5. Clogging and wear of nozzles due to silt, sand, or algae; need good filtration.
6. Crop problems: wetting of leaves may promote fungal disease and burn leaves if water is saline; flowers and fruit may be damaged.
7. Unsuitable soil: low-infiltration clay causes surface ponding and runoff.
8. Uneven distribution from overlapping and bad spacing; poor design can leave dry spots.
9. Skills and maintenance: lack of skilled operators and spare parts; shifting of portable lines takes labour and may damage crops.
10. Damage to pipes by animals, vehicles, sun, and theft.

Layout and wetting pattern

The four sprinklers are placed 20 m apart, i.e. at the centres of the four 20 m x 20 m quarters of the plot, at (10,10), (30,10), (10,30) and (30,30) m. Each wets a circle of radius 10 m.

 40 m +--------------------+
      |  .--.      .--.    |
      | ( S1 )    ( S2 )   |
      |  '--'      '--'    |
      |  .--.      .--.    |
      | ( S3 )    ( S4 )   |
      |  '--'      '--'    |
   0  +--------------------+
      0       20        40 m
   circles touch only at one point;
   the 4 corners and centre are dry

Calculation

$$\begin{aligned} A_{plot} &= 40\times 40 = 1600\ \text{m}^2\\ A_{wetted} &= 4\times\pi (10)^2 = 1256.6\ \text{m}^2\\ \text{Coverage} &= 1256.6/1600 = 78.5\%\\ \text{Dry area} &\approx 343\ \text{m}^2\ (21.5\%) \end{aligned}$$

Comments

• The circles only touch; there is no overlap, so the centre of the plot and the four corners and the strips between circles stay dry.
• The depth of water is highest near each sprinkler and falls to zero at the 10 m edge, so distribution is non-uniform (poor uniformity coefficient).
• About 21% of the plot gets no water.

Improvement

1. Reduce the spacing to about 50-60% of the wetted diameter (20 m): 10-12 m square spacing, giving overlap of the neighbouring circles. A 13.3 m spacing (3 x 3 = 9 sprinklers) or 10 m (4 x 4 = 16 sprinklers) would cover the plot properly.
2. Or increase the throw radius to at least $10\sqrt{2} = 14.14$ m (about 14.2 m) with the same four sprinklers, so the circles meet at the centre of the plot.
3. Use a triangular pattern for better uniformity, and place sprinklers along the edges and at the corners in the rectangular layout.
4. Avoid operating in high wind.