Reservoir water loss from evaporation and plant uptake: what level sensor data shows across dry and wet season

Key takeaways

In Bangkok dry season (February to April), level sensor logs show total daily reservoir volume loss averaging 4.1 percent of reservoir capacity per day across monitored NFT systems -- roughly 1.8x the loss rate recorded during the wet season months of September and October. Plant Mood grow log data, 2024.

Evaporation accounts for an estimated 35 to 55 percent of total daily water loss in an uncovered reservoir at Bangkok ambient temperatures above 33 degrees Celsius, based on level drop patterns logged during dark hours when plant uptake is negligible.

EC rises faster than pH during high-evaporation periods because evaporation removes pure water while leaving dissolved salts behind. A reservoir that reads correct EC at 7am can read 0.4 mS/cm higher by 3pm on a dry-season afternoon without any nutrient addition.

A fixed top-off volume calibrated for wet season conditions will under-compensate during dry season by enough to push EC outside the crop's tolerance band within 48 hours in a 40-litre reservoir.

Level sensors logging at 15-minute intervals are the minimum resolution needed to separate evaporation loss (visible as steady overnight decline) from plant uptake spikes (visible as stepped drops during peak light hours).

TL;DR

Most hydroponic guides treat reservoir top-off as a single variable. In Bangkok it is two variables -- evaporation and plant uptake -- that move independently and shift with the season. The dry season nearly doubles total daily water loss compared to wet season, and the evaporation share climbs enough that topping off without distinguishing the source will throw EC into drift. The sensor data makes this separable. Most growers never look.

The reservoir that looked fine until it wasn't

April. 38 degrees Celsius ambient in the grow space at 2pm. The NFT channel is running, the pump is healthy, the plants look fine. The reservoir level sensor shows a 14 percent volume drop since the previous morning's top-off -- roughly double what the same system was losing in October. EC reads 2.6 mS/cm against a target of 1.8. The pH is still inside range, which is the part that throws you: the plants are not showing deficiency symptoms yet, and pH looks normal, so the obvious checks pass. But the nutrient solution is now concentrated enough that continued uptake is going to pull the wrong ratios. Two days from that reading, the basil tips started showing interveinal chlorosis consistent with iron lockout at elevated EC.

The problem was not a dosing error or a contaminated batch. It was a top-off schedule built around wet-season evaporation rates applied unchanged through the driest month of the year. The sensor log showed exactly what happened, in 15-minute increments, going back three weeks. Evaporation had been climbing steadily since February. Nobody adjusted the top-off volume because there was no process for checking whether it needed adjusting.

What the research says about evaporation in controlled growing environments

Papadimitriou et al., Biosystems Engineering, 2021

In greenhouse NFT systems, evaporation from open reservoir surfaces and exposed channel sections accounted for 18 to 42 percent of total system water loss depending on ambient vapour pressure deficit, with higher VPD conditions producing proportionally greater evaporative loss relative to transpiration.

Bangkok dry season VPD regularly exceeds 2.5 kPa in uncontrolled indoor spaces. At those levels, the 42 percent upper bound from this research is not a worst case -- it is closer to baseline for an uncovered reservoir near a window or in a space with direct airflow.

Savvas & Gruda, European Journal of Horticultural Science, 2018

Nutrient solution concentration increases caused by evaporative water loss are non-linear relative to uptake-driven concentration increases because evaporation removes solvent without altering solute ratios, while plant uptake selectively removes certain ions faster than others.

This matters for EC management. When evaporation is the dominant loss mechanism, EC rises uniformly across all ions. When uptake is dominant, ion ratios shift and EC alone is insufficient to characterise the solution state. In high-evaporation Bangkok conditions, EC trending up uniformly is a signal that evaporation is outpacing uptake -- not that the crop is under-watered.

Priva Horticulture technical notes on VPD and reservoir management (2022)

Systems operating without reservoir covers in high-VPD environments showed evaporation rates 2.3x higher than covered systems under identical ambient conditions, with the differential increasing as ambient temperature exceeded 32 degrees Celsius.

A floating cover or even a close-fitting lid on the reservoir is not a minor optimisation in Bangkok. It is a meaningful intervention. Across a 40-litre reservoir operating for 12 hours at 35 degrees Celsius ambient, the difference between covered and uncovered can be 3 to 4 litres of evaporative loss per day -- enough to shift EC measurably by mid-afternoon.

The numbers that change when the season does

4.1 percent of reservoir volume lost per day during Bangkok dry season versus 2.3 percent during wet season

Logged across three NFT systems, 40 to 60 litre reservoirs, lettuce and basil crops, February to October 2024. Plant Mood grow log data, 2024. The difference is not marginal. It is the kind of gap that makes a wet-season top-off routine actively wrong by March.

Overnight level drop (10pm to 6am) averaged 0.9 percent of reservoir volume during dry season versus 0.4 percent during wet season

Plant Mood grow log data, 2024. With the lights off and plant uptake near zero, the overnight drop is almost pure evaporation. That 0.5 percent difference across a 50-litre reservoir is 250ml of water lost to the air every night before the lights even come on -- and before any top-off decision gets made.

EC readings taken at 6am versus 3pm during dry-season days with no top-off showed an average 0.38 mS/cm increase across 18 measurement days

Plant Mood grow log data, 2024. The afternoon peak EC was consistently higher than the morning reading even though no nutrients were added. The mechanism is evaporation concentrating the solution through the day.

Ambient relative humidity in the grow space dropped from an average of 74 percent in September to 54 percent in March

Plant Mood environmental sensor log, 2024. The humidity swing is the driver. Lower RH means higher vapour pressure deficit, which accelerates evaporation from any exposed water surface.

What the level sensor logs actually showed

Evaporation produces a slow, near-linear decline across all hours -- steeper when ambient temperature is higher, but continuous. Plant uptake produces stepped drops that cluster during peak light hours and flatten at night. When you graph 24 hours of level data at 15-minute resolution, these two patterns do not overlap cleanly, but the overnight window from 10pm to 6am is clean enough to isolate evaporation with reasonable confidence. Across dry-season cycles, that overnight window consistently showed 55 to 70ml of loss per hour in uncovered 50-litre reservoirs at 33 to 36 degrees Celsius ambient.

Evaporation and uptake produce distinguishable signatures in 15-minute level data

A grower who tops off once a day based on total level drop has no idea what fraction of that drop was evaporation versus uptake. Those two causes require different responses. Evaporation should be compensated with plain water. Uptake-driven loss may need dilute nutrient solution depending on where EC sits. Treating them identically is one of the more reliable ways to push EC out of range slowly enough that you don't notice until the crop is already stressed.

The top-off volume in use was calibrated during October, which sits inside the wet season window. When that same volume was applied unchanged through February and March, the daily level deficit compounded. By day three of the dry-season cycle, the reservoir was running 8 to 12 percent below target volume before the daily top-off was triggered, which meant the plants were pulling from a more concentrated solution for most of the day.

Dry-season evaporation rates exceeded the default top-off volume by an average of 31 percent

The failure mode here is not dramatic. The system does not crash. EC creeps. The plants look fine for longer than they should. The grower who checks EC once a day in the morning -- after overnight evaporation has already raised concentration -- will see a number that looks manageable. The grower who checks at 3pm will see something different. We started logging EC at three fixed points per day specifically because of this pattern.

A basic floating polystyrene cover cut to fit the reservoir surface, with cutouts for the pump line and return, was enough to produce this reduction. The cover does not eliminate evaporation from channel surfaces or any exposed tubing, but the reservoir surface is the largest single evaporation area in a typical NFT setup. The 1.4-litre daily difference is not a rounding error across a crop cycle -- it is 42 litres over a 30-day cycle, which is more than three-quarters of the reservoir capacity.

Covering the reservoir reduced dry-season daily volume loss by an average of 1.4 litres across 50-litre systems

This is the intervention with the best ratio of effort to outcome we have found for reservoir water management in Bangkok conditions. It costs almost nothing and the sensor data shows a clear reduction. The growers who dismiss reservoir covers as unnecessary are, in most cases, running covered grow spaces with lower VPD than a Bangkok apartment. The advice does not transfer.

Plant Mood grow log data, 2024

What most Bangkok growers are not measuring

When EC climbs without a clear nutrient addition, the default assumption is that something went wrong with the mixing or the concentrate. That assumption sends growers in the wrong direction -- checking their stock solution, recalibrating their dosing pump, questioning the nutrient brand. The sensor data points somewhere simpler: evaporation removed water and left the salts behind. The fix is plain water, not a troubleshooting spiral. The diagnostic is straightforward if you log level alongside EC: if level dropped and EC rose without any nutrient addition, evaporation is the most likely mechanism. Most growers do not log both at sufficient resolution to see this.

Treating EC drift as a dosing problem when it is an evaporation problem

A top-off routine established in October is calibrated to October conditions. It will under-compensate by March. The grower who set it and forgot it is not making a mistake -- they are just not treating top-off as a variable that needs seasonal recalibration. This is structurally inevitable for anyone running more than a few systems without automated level control: there is no natural forcing function that prompts a review until something goes wrong. The first signal is usually an EC reading that won't come down, not a visible plant problem.

Applying the same top-off volume across seasons without review

Humidity data is collected by almost everyone who has a basic environmental sensor. Almost nobody uses it to adjust reservoir management. The relationship is direct: lower ambient RH means higher VPD means faster evaporation from exposed water surfaces. A humidity reading below 55 percent in the grow space during daylight hours is a signal to check whether the current top-off volume is still appropriate. This is not an automated step in any grow guide I have seen written for Bangkok conditions.

Ignoring the grow space humidity reading as a reservoir management input

A grower moving from 10 to 20 plants assumes water loss scales linearly with plant count. It does not. Evaporation scales with surface area and ambient VPD, not plant count. Uptake scales with plant biomass and light intensity. When you double the plant count but keep the same reservoir surface area, you get more uptake but roughly the same evaporation -- which changes the ratio between the two loss mechanisms and should change the top-off strategy. Almost no one accounts for this during a scaling step.

Not distinguishing plant uptake rate from total water loss when scaling up

The complaints that point at the real problem

My EC keeps climbing even though I haven't changed anything in my nutrient mix.

In most cases this is evaporation-driven concentration, not a dosing error. The nutrient mix has not changed -- the water-to-salt ratio has, because water left via evaporation and the salts stayed. The fix is plain water top-off sized to the evaporation rate, which requires knowing what the evaporation rate actually is. Without level logging, this is a guess. With it, it is a number.

I top off every morning but the plants still look slightly stressed by late afternoon.

A morning top-off compensates for the previous day's loss. If evaporation is high enough -- and during Bangkok dry season it is -- the reservoir concentration climbs again through the afternoon before the next top-off. The plants are spending the hottest part of the day pulling from a more concentrated solution than they experienced at 7am. A single daily top-off is not enough resolution for dry-season Bangkok conditions in an uncovered system. Either split the top-off into morning and afternoon additions, or cover the reservoir to slow the concentration climb.

My system worked perfectly for six months and then suddenly started having EC problems in February.

February is when Bangkok dry season shifts into its most aggressive phase. Ambient humidity drops, temperature climbs, and evaporation rate from the reservoir accelerates. The system did not change. The environmental load on it did. This is not an intuitive pattern if you are not logging environmental data alongside reservoir data. The grower who sees it as a sudden failure will look for a sudden cause -- contamination, equipment fault, batch error -- and miss the slow seasonal shift underneath it.

I covered my reservoir and it made almost no difference to the EC drift.

A reservoir cover addresses reservoir surface evaporation. NFT channels, return lines, and any exposed tubing or grow medium surface also contribute to total system evaporative loss. In a system where channel surface area is large relative to reservoir area -- common in longer NFT runs -- covering only the reservoir removes a fraction of the total evaporation surface. The fix requires either covering exposed channel sections as well, or accepting that the cover is a partial intervention and adjusting the top-off calculation accordingly.

The assumption that produces the wrong EC outcome

The logic is not wrong -- it is incomplete. EC rising means the solution is more concentrated than target. Plain water dilutes it back toward target. EC falling means the opposite. The problem is that the correction volume required depends on whether the EC shift was caused by evaporation or by plant uptake, and those two mechanisms produce different ion profiles in the remaining solution. Evaporation raises EC uniformly across all ions. Selective plant uptake raises EC while also shifting the ion ratio -- the plant pulls calcium and potassium faster than it pulls sulphur, so the ratio in the remaining solution drifts even if the total EC reading looks correctable with plain water. If you apply plain water to a solution whose EC rose because of selective uptake rather than evaporation, you bring EC down but leave the ion imbalance intact. Over multiple correction cycles this compounds. In practice, for most growers running standard crop varieties in balanced nutrient solutions, the error from not distinguishing the two mechanisms is small enough to absorb. But in high-evaporation Bangkok conditions where both mechanisms are running hard simultaneously, the compounding matters more than it does in a temperate climate where evaporation is minor.

Across cycles where we logged both level change and EC change at 15-minute resolution, the overnight period (lights off, plant uptake near zero) showed EC rising in direct proportion to volume lost -- consistent with pure evaporation concentrating the solution. During peak light hours, EC sometimes held steady or dropped slightly even as volume fell, which indicates that plant uptake was pulling ions faster than evaporation was concentrating them. A flat or declining EC while the level sensor shows volume dropping is not a signal that everything is fine -- it is a signal that uptake is high, which has its own implications for nutrient replenishment timing. Plant Mood grow log data, 2024.

If EC is rising, add plain water. If EC is falling, add nutrient solution. This is the standard correction logic and it is what most guides recommend.

How to set up reservoir management for Bangkok seasonal conditions

Geert Warmenbol

Install a float-based or ultrasonic level sensor in the reservoir and log at 15-minute intervals minimum. Without this, all subsequent steps are estimates. The sensor does not need to be expensive -- a basic float switch with a data logger is sufficient to capture the overnight evaporation signature. What matters is logging frequency, not hardware cost.

A baseline record of daily volume loss that can be separated into overnight (evaporation-dominant) and daytime (mixed evaporation and uptake) components within two weeks of logging.

Establish a seasonal calibration review at the start of each season transition: late January (before dry season peaks), late May (monsoon onset), and late October (post-monsoon settling). At each review, pull the overnight average level drop from the previous two weeks and compare it against the same metric from the prior season. Adjust the daily top-off volume to match the current evaporation rate, not the historical one.

Top-off volumes that track seasonal evaporation rather than lagging behind it. EC drift caused by evaporation undershoot should be eliminated or substantially reduced.

Cover the reservoir. A floating polystyrene sheet cut to fit, with pump and return line cutouts, is sufficient. This is not an aesthetic intervention -- the sensor data supports it. Fit the cover at the start of dry season and remove it if the grow space becomes humidity-saturated during peak wet season months, when evaporation is low enough that cover-induced condensation becomes a more relevant concern.

Reduction in daily reservoir volume loss of 1 to 1.5 litres per 50-litre reservoir during dry season months, based on logged comparison between covered and uncovered periods.

Log EC at three fixed daily timepoints: before the morning top-off, at peak light hours (typically 1pm to 3pm), and at lights-off. The difference between the morning and peak reading, in the absence of any nutrient addition, isolates the intra-day evaporation-driven concentration effect. A consistent mid-afternoon EC reading more than 0.3 mS/cm above the morning reading is a signal that the top-off volume or timing needs adjustment.

Early detection of evaporation-driven EC drift before it crosses crop tolerance thresholds. The pattern typically becomes visible two to three days before plant stress symptoms appear.

For systems where a single daily top-off is logistically fixed, bias the top-off to mid-afternoon rather than morning. A morning top-off compensates for overnight evaporation but does not address the afternoon concentration climb. A mid-afternoon top-off addresses the period of highest evaporation-driven concentration and leaves the overnight period -- where evaporation is slower -- as the remaining gap. This does not eliminate the need for a correctly sized volume, but it changes the distribution of EC exposure across the day.

Lower peak EC during high-light hours, which is the period of highest plant metabolic activity and nutrient uptake demand.

Step 2 -- the seasonal calibration review. Growers establish a top-off routine, it works, and they leave it. The gap between wet-season and dry-season evaporation rates is large enough in Bangkok that this is not a minor optimisation. It is the difference between a top-off volume that matches the system and one that structurally under-compensates for four months of the year.