Irrigation Study Guide

📖 Core Concepts Irrigation – Controlled application of water to land for crops, landscapes, or lawns. Drainage – Removal of excess surface or sub‑surface water; often paired with irrigation in water‑balance studies. Supplementary vs. Full Irrigation – Supplementary adds to rainfall; full supplies most or all crop water needs (typical in arid zones). Water‑Use Efficiency (Field Water Efficiency) – % of applied water that is actually transpired by the crop. Micro‑Irrigation – Low‑pressure, localized delivery (drip, subsurface drip, micro‑sprinklers). Sprinkler Irrigation – Overhead distribution by high‑pressure sprinklers (center‑pivot, lateral‑move). Surface (Gravity) Irrigation – Water moves by gravity across the field surface (furrow, border, basin, flood). --- 📌 Must Remember Field Water Efficiency = (Water transpired ÷ Water applied) × 100 %. Drip irrigation efficiency: typically 80 %–90 % when managed correctly. Surface irrigation: lowest capital cost, highest energy savings, but low water‑application efficiency. Over‑irrigation → deep drainage, water‑table rise, salinity, nutrient leaching. Under‑irrigation → poor soil‑salinity control; periodic leaching required. Wastewater reuse benefits: nutrient supply (N, P, K), lower cost, steady supply. Risks: pathogens, heavy metals, pharmaceuticals. Historical growth: 8 M ha (1800) → 94 M ha (1950) → 235 M ha (1990); irrigated land now produces 30 % of global food. --- 🔄 Key Processes Designing a Drip System Map field, locate root zones. Choose emitter flow rate (e.g., 2 L h⁻¹) and spacing to match crop water demand. Install main line, laterals, and emitters; pressure‑regulate. Schedule runs based on soil moisture sensors or ET‑crop calculations. Sprinkler Scheduling (ET‑based) Determine crop evapotranspiration (ET₍c₎) → ET₍c₎ = ET₍0₎ × Kc. Subtract effective rainfall. Apply water in pulses that keep soil moisture within the allowable depletion range (usually 30–50 %). Surface Irrigation Runoff Control Level field to uniform slope (≤0.2 %). Set inlet flow rate so that advance front moves uniformly. Shut off when the front reaches the far end; allow infiltration time. Leaching for Salinity Control Apply extra water (leaching fraction, LF) = (EC₍w₎ / EC₍e₎) − 1, where EC₍w₎ = irrigation water EC, EC₍e₎ = acceptable soil EC. --- 🔍 Key Comparisons Drip vs. Sprinkler – Drip: high efficiency, low runoff, good for row crops; Sprinkler: quicker coverage, suitable for cereals, higher energy use. Center‑Pivot vs. Lateral‑Move – Pivot: best on flat, large fields; Lateral‑Move: cheaper, works on irregular or hilly terrain. Full vs. Supplementary Irrigation – Full: meets 100 % crop water need; Supplementary: fills gaps between rainfall events. Surface vs. Micro‑Irrigation – Surface: low cost, low control; Micro: higher capital, precise water placement. Groundwater vs. Surface Water Supply – Groundwater: reliable year‑round, risk of overdraft; Surface: dependent on seasonal flow, may require storage. --- ⚠️ Common Misunderstandings “All drip systems are >90 % efficient.” – Efficiency drops if emitters clog, flow rates are too high, or runoff occurs on sloped terrain. Confusing field water efficiency with overall irrigation system efficiency. The former ignores losses in conveyance and distribution. Assuming more water always increases yield. Over‑irrigation can reduce yield via salinity and oxygen depletion. Treating wastewater as a “free fertilizer.” Nutrient content varies; heavy metals or pathogens may require treatment. --- 🧠 Mental Models / Intuition Water‑Balance Box: Input = Rainfall + Irrigation + Groundwater inflow – (Evapotranspiration + Runoff + Deep drainage). Keeping the box “filled but not overflowing” ensures optimal water use. Root‑Zone Target: Visualize a thin “sweet‑spot” zone around roots; micro‑irrigation aims to wet only this zone, while surface irrigation wets the entire profile indiscriminately. --- 🚩 Exceptions & Edge Cases High‑sodium or saline water → may require blending with fresher sources or leaching; otherwise it damages soil structure. Very shallow water tables → subirrigation can be advantageous, but risk of water‑logging if not monitored. Steep slopes – surface irrigation becomes inefficient; micro‑irrigation or terracing is preferred. Cold climates – drip lines can freeze; insulated or buried lines are needed. --- 📍 When to Use Which Choose Drip when: High‑value, row‑crop (vegetables, orchards). Water scarce or expensive. Soil is sandy/low water‑holding capacity. Choose Sprinkler (Center‑Pivot) when: Large, flat fields of cereals or grasses. Need rapid coverage and flexibility in application rates. Choose Surface (Furrow/Border) when: Capital is limited and terrain is gently sloping. Crop tolerance to uneven moisture is high (e.g., rice). Use Wastewater only after: Pathogen removal (e.g., chlorination, UV). Heavy‑metal testing meets crop‑safety standards. --- 👀 Patterns to Recognize Ponding at field edges → excess inlet flow → surface runoff losses. Uniform green canopy but low yield → possible deep percolation or nutrient leaching. Rising water table + salt crusts → over‑irrigation + inadequate drainage. Spotty emitter wetting → clogging or pressure loss in drip lines. --- 🗂️ Exam Traps Trap: “Field water efficiency = (Water applied ÷ Water transpired) × 100.” – The fraction is reversed; it should be transpired ÷ applied. Trap: Assuming “full irrigation” always uses more water than “supplementary.” In arid zones, supplemental may equal full if rainfall is nil, but the term still means adding to rain, not replacing it. Trap: Selecting “center‑pivot” for a hilly orchard because it’s “modern.” Terrain slope makes pivots inefficient; lateral‑move or drip is better. Trap: Believing wastewater automatically reduces fertilizer costs. Nutrient content is variable; over‑application can cause toxicity. ---