Water and Irrigation for Farms in Thailand

Water is the input a new farm gets wrong most often, and the one where the mistake is hardest to see. A nutrient problem shows on a leaf. A watering problem shows as a plant that is simply not doing well, for weeks, with no obvious cause.

This guide is written for growers starting out on a farm in Thailand. It covers what the three Thai seasons do to your watering, how outdoor, greenhouse and indoor growing differ, and the mistakes that cost new growers the most. Three companion pages go deeper on water quality, on watering by growing medium, and on building a drip system.

Every threshold here is attributed to a published source. Where a number comes from our own calculation on published data rather than from the source directly, it says so. Nothing is included that we could not trace.

Irrigation water quality, pH and EC

What to measure in your water and what the numbers mean: pH, EC, alkalinity, hardness, sodium and chloride, with thresholds from FAO and university sources. Includes how Thai municipal supply, borewell water, canal water and harvested rain actually compare, when reverse osmosis is justified, and how to keep a meter honest. Start here if you have never had a water test done.

Watering by growing medium

Why the same plant needs a completely different schedule in soil, in coconut coir and in a passive system. How to tell overwatering from underwatering, why coir needs buffering and calcium-magnesium, and how much runoff you should be seeing.

Drip irrigation setup and maintenance

The parts of a drip system and what each one prevents, filtration grade for your water source, the formulas for working out emitter flow and run time from your own numbers, uniformity checks, and the flushing and treatment routine that keeps a system working into its third season. Plus rainwater harvesting for the wet season.

The three Thai seasons, and what they do to watering

Irrigation guidance written for temperate countries assumes four seasons and a winter in which plants stop growing. Neither applies here, and using that framing will mislead you.

The Thai Meteorological Department defines three seasons: hot from mid-February to mid-May, rainy from mid-May to mid-October, and cool from mid-October to mid-February. These are climatological boundaries; the department issues a dated announcement each year and the actual onset moves. The 2569 rainy season was declared on 15 May 2026. Onset is declared when rain covers more than 60 percent of upper Thailand for several consecutive days, wind at about 1.5 km altitude turns southwesterly, and wind at 10 km and above turns easterly.

How much the water demand actually changes. The Royal Irrigation Department publishes reference evapotranspiration by station, calculated by the Penman-Monteith method from 30 years of data (1981–2010) across 120 meteorological stations. Selected monthly values in millimetres per day:

Station Hot season peak (April) Rainy season low (September) Cool season (December)
Bangkok (Don Mueang) 5.54 3.95 4.04
Chiang Mai 6.12 3.67 3.03
Khon Kaen 4.97 3.61 3.63
Ubon Ratchathani 5.03 3.43 4.22
Nakhon Ratchasima 4.64 3.36 3.41

Reading across those rows: peak demand is roughly 1.4 to 2.0 times the seasonal minimum, depending where you are — Chiang Mai swings from 3.03 to 6.12 mm/day, a factor of two. That ratio is our arithmetic on the department's published figures. A farm running one fixed schedule year-round is therefore either drowning plants in the cool season or starving them in April. Kasetsart University research puts Thailand's average annual reference evapotranspiration at 1,434 mm, ranging from 1,200 mm at Umphang to 1,683 mm at Chai Nat.

The practical consequence by season:

  • Hot season (mid-February to mid-May) — highest demand of the year, peaking in April. The Thai Meteorological Department classifies 35.0–39.9 °C as hot and 40 °C and above as very hot; Khon Kaen's record high is 43.1 °C, recorded in April. Containers dry fastest here and small containers can go from wet to critically dry within a day. This is also when municipal water in Bangkok can turn brackish, discussed below.
  • Rainy season (mid-May to mid-October) — demand falls, and the problem inverts. Drainage and preventing waterlogged roots now matter more than supplying water. Rain also does your leaching for you in open ground.
  • Cool season (mid-October to mid-February) — lowest demand. Reduce volume rather than frequency first. Thai nights are mild by temperate standards — the department classifies 16.0–22.9 °C as cool — so growth does not stop, and neither should watering.

Humidity is the quiet factor. Thailand runs high relative humidity year-round: Bangkok's annual mean is around 73 percent, rising to about 79 percent in September; Hat Yai reaches nearly 87 percent in November. High humidity slows transpiration, which means a plant uses less water than the temperature alone suggests, and it keeps foliage wet for longer after any overhead watering. Both push toward watering at the base rather than over the canopy.

Outdoor growing

Outdoors, the season decides which problem you are solving.

In the rainy season, drainage is the job. Roots need oxygen as well as water; when soil stays saturated, the air spaces fill and root function is limited. Thailand's Department of Agricultural Extension gives a concrete specification for shallow-rooted vegetables: raise beds not less than 30 cm. It also advises increasing organic matter to improve soil structure and air-filled porosity.

The Northeast receives a mean annual rainfall of 1,470.8 mm, measured across 27 meteorological stations over 30 years (1993–2022), with a regional spread of roughly 1,100 to 1,900 mm and the wettest months in August and September. National rainfall ranges from about 1,000 to nearly 5,000 mm a year depending on location. Plan drainage for your own rainfall, not a national average.

Practical measures: raised beds; drainage channels kept clear before the season starts, not during it; pumps ready if your ground is low-lying. After flooding recedes, the Department of Agricultural Extension advises avoiding trampling and machinery on wet ground because of compaction, pruning damaged growth, loosening soil to restore root aeration, and treating for root rot.

In the hot season, evaporative demand is the job. Water early morning or late afternoon rather than midday. Mulch reduces surface evaporation and keeps root-zone temperature down. Increase volume per irrigation before increasing frequency, so that water reaches full root depth — frequent shallow watering keeps roots near the surface where they are most exposed.

In the cool season, reduce. Demand falls by a third to a half depending on region. Keep watering deeply but extend the interval. This is the season when overwatering does its quietest damage, because the plant's reduced uptake is easy to miss.

Greenhouse and net house

Protected structures in Thailand are usually plastic-roofed houses or net houses, and the two behave quite differently for watering.

A net house excludes insects and reduces light intensity but does not exclude rain. Watering is closer to outdoor practice, with the important exception that shading lowers evaporative demand, so applying the same schedule as an adjacent open field will overwater.

A plastic-roofed house excludes rain entirely, and that changes the chemistry as much as the schedule. Nothing leaches unless you leach it. Salts from your water and fertiliser accumulate with every irrigation, which is exactly the situation FAO Irrigation and Drainage Paper 29 describes its guidelines as applying to most directly — the paper notes its thresholds assume a climate "where rainfall does not play a significant role" in meeting crop water needs. Under a plastic roof in Thailand you are, chemically speaking, farming in an arid climate. Leaching fraction and runoff monitoring become essential rather than optional.

Evaporative cooling — a wet pad at one end and fans at the other — works by evaporating water into the incoming air, which cools it and raises humidity. Its effectiveness depends on how dry the incoming air is, so in Thailand it performs much better in the hot season, when humidity is comparatively lower, than in the rainy season, when the air is already near saturation. Two consequences for watering: the system consumes water itself and that must be in your budget, and raised humidity reduces plant transpiration, so crop water use drops while the cooling system runs.

In any covered structure, prolonged leaf wetness is a risk in a climate with high humidity and warm nights. Water at the base. If overhead watering is unavoidable, do it early enough that foliage dries before nightfall.

Indoor growing

Indoors you control everything, which means nothing is decided for you and every setting interacts with the others.

Water use indoors is driven by transpiration, and transpiration is driven by the difference between the moisture the air can hold at leaf temperature and what it currently holds — not by temperature alone. This is why identical rooms at the same temperature can use very different amounts of water: the one at lower humidity transpires faster. Raising temperature without raising humidity increases water demand; raising humidity at the same temperature reduces it.

Airflow matters more than new growers expect. Still air lets a humid boundary layer sit against the leaf surface, which slows transpiration and therefore slows the transport of water and dissolved nutrients up through the plant. Moving air breaks that layer. It also dries leaf surfaces, which matters in a warm room.

Root-zone temperature deserves particular attention in a tropical climate, because warmth and root disease are linked. Research on soilless systems reports that roots colonised by Pythium at cooler temperatures can stay symptomless yet "develop severe visible decay within hours when temperature rises to 24–28 °C", with peak virulence of one species above 30 °C. The same literature reports growth suppression in tomato and cucumber at dissolved oxygen of 1 to 3 mg/L, and a commercial minimum recommendation of 5 mg/L in hydroponic production — while noting honestly that no crop-specific threshold is actually established.

In practice this means a warm, wet, still root zone is the combination to avoid. Keep reservoirs and containers out of direct heat, do not leave the medium saturated, and move air.

Because indoor growing is usually soilless and in containers, the two companion pages on growing medium and water quality carry most of the detail that matters indoors.

Six mistakes that cost new growers the most

1. Watering too often. This is the most common and the most damaging, and it is a different mistake from watering too much at once. Frequent light watering keeps the medium permanently wet, so air never returns to the root zone and roots cannot function. The confusing part is that the plant then wilts — because the roots are damaged — and the instinctive response is to water more.

Signs: medium stays wet long after watering; container stays heavy; growth slows for no clear reason; wilting despite wet medium; sour smell from the medium.
Fix: check moisture at root depth, not the surface, before every watering. In containers, lift the pot — the weight difference is obvious once learned. Extend the interval and keep the volume, rather than the reverse.

2. Watering too little, too shallowly. The opposite failure, and it produces a shallow root system that cannot buffer a hot April day.

Signs: medium dry at root depth; pots light; afternoon wilting with overnight recovery; crisp dry leaf edges; in coir or peat, the medium pulling away from the container wall — after which water runs down the gap and straight out without wetting anything.
Fix: water until the full root depth is wet. If the medium has shrunk from the wall, rewet slowly in several small applications to let it reabsorb.

3. Never measuring what drains out. Feed EC tells you what you dosed. Runoff EC tells you what the root zone is doing, which is the question that matters. Without it you are guessing.

Fix: compare the two. Runoff EC higher than feed EC means salts are concentrating — increase volume or leaching fraction. Lower means over-leaching and wasted fertiliser. Roughly equal is the target. Published leaching fractions for container and soilless growing converge on 15 to 30 percent; we found no primary source supporting the 50 percent figure that circulates widely.

4. Adjusting pH in the wrong order. Adding nutrients changes the pH of the solution, so pH set before the fertiliser goes in is not the pH the plant receives.

Fix: fill with water, add and mix nutrients fully, then measure and adjust pH, then check again after a few minutes. Where acid is used to treat alkaline water, UMass advises that acid be "always injected prior to the addition of fertilizer or other chemicals" — note that this is a separate step from final pH adjustment, and the two are often confused. And always add acid to water, never water to acid.

5. Ignoring alkalinity because pH looks fine. The mistake that takes longest to notice. pH is an instantaneous reading; alkalinity is how much acid-neutralising capacity you dose with every irrigation, and it accumulates. Purdue Extension puts it plainly: "irrigating your crops with water high in alkalinity has the same effect as adding lime to the substrate". Water at pH 8.0 with very low alkalinity is harmless; water at pH 7.2 with high alkalinity will push substrate pH upward week after week. Most basic water reports do not include it unless you ask.

Fix: get alkalinity measured, and ask whether it is reported as calcium carbonate or as bicarbonate — the same water reads 1.22 times higher on the bicarbonate basis. Details on our water quality page.

6. Trusting a borewell without testing it. In the Northeast this is a specific, documented risk rather than general caution. The Khorat Plateau is underlain by the Maha Sarakham Formation, which contains rock salt; groundwater dissolves it and carries the salt upward. In 189 monitoring wells sampled in the central Huai Luang Basin, Udon Thani, total dissolved solids ranged from under 60 to 55,800 mg/L — about 75 percent of samples below 1,000 mg/L, but around 7 percent above 10,000 mg/L. Against FAO's classification, that top group is several times the severe restriction threshold and is not irrigation water at any concentration. Neighbouring wells can differ enormously.

Fix: test before you commit a season to a well, and test again if a crop starts failing for no visible reason.

A seventh, for anyone in Bangkok: tap water is not constant. In the dry season, spring tides push seawater up the Chao Phraya to the waterworks intake. In January 2020 the Metropolitan Waterworks Authority publicly announced chloride exceeding the 250 mg/L standard across around a dozen branch areas for periods of six to twelve hours. That happens in the hot season, exactly when irrigation demand peaks.

Downloads

Thai and English editions of both documents are being prepared.

Related guides

Sources

  • Thai Meteorological Department, season definitions and weather criteria, via the Thai Government Portal. thailand.go.th
  • Government Public Relations Department, announcement of the 2569 rainy season onset. prd.go.th
  • Royal Irrigation Department (2554). Reference crop evapotranspiration by the Penman-Monteith method. rid.go.th
  • Vudhivanich, V. (1996). Monthly Potential Evapotranspiration of Thailand, Kasetsart J. (Nat. Sci.) 30(3):392.
  • Department of Agricultural Extension (2023). Growing and caring for vegetables in the rainy season. doae.go.th
  • Department of Agricultural Extension (2023). Orchard management during and after flooding. doae.go.th
  • Influence of El Niño Southern Oscillation on precipitation variability in Northeast Thailand, Results in Engineering (2024).
  • Arai, S. et al. (2019). Characteristics of Gridded Rainfall Data for Thailand from 1981–2017, Engineering Journal 23(6):461.
  • Ayers, R.S. & Westcot, D.W. (1985). Water Quality for Agriculture, FAO Irrigation and Drainage Paper 29 Rev. 1. fao.org
  • Lopez, R.G. et al. (2010). Alkalinity Management in Soilless Substrates, Purdue Extension HO-242-W. purdue.edu
  • Cox, D. Water Quality: pH and Alkalinity, UMass Amherst. umass.edu
  • Pholkern, K. et al. (2019). Water 11(2):241. doi.org
  • Wongsomsak, S. (1986). Salinization in Northeast Thailand, Southeast Asian Studies 24(2):133.
  • Metropolitan Waterworks Authority. Annual Water Quality Report 2566. mwa.co.th
  • Dissolved oxygen limitation and Pythium root rot in strawberry NFT systems (2026), Frontiers in Plant Science. nih.gov
  • WMO climate normals 1991–2020 for Thai stations, supplied by the Thai Meteorological Department.