Agricultural Weather Station Evaporation Data: Account for Rain and Refilling

Agricultural weather station evaporation data should describe water lost from an observed surface, not every fall in an evaporation tank's level. Rain, refilling, drainage and leakage all affect that level. NiuBoL weather and water-level instruments can support a useful record when the logger accounts for those events and keeps pan evaporation separate from soil moisture and crop water use.

Define the water balance before choosing the display

An evaporation sensor observes a water-level change in a pan or tank. Under stable conditions with no other water transfers, a decrease can be attributed to evaporation. In an open field, those conditions must be checked rather than assumed. A daily level difference alone can underestimate evaporation during rain or overestimate it after unnoticed drainage.

Define one observation period and use the same boundary for the level readings, rain record and maintenance log. If rain is reported by calendar day but the tank is read between morning visits, the correction may cover different hours. A plausible daily total does not reveal that mismatch on its own.

Record water entering and leaving the pan

For a fixed-area pan, express each transfer as a water depth over its surface. With consistent signs, evaporation equals starting level minus ending level, plus water inputs, minus non-evaporative outputs. The calculation requires the included transfers to be known. Unmeasured overflow or leakage leaves an unresolved balance, not a trustworthy evaporation number.

Use an explicit example to test the logger calculation

Consider a hypothetical period starting at 100 mm and ending at 97 mm, with 4 mm of rain entering the pan, 3 mm of refill water and 2 mm intentionally removed. With no other losses, evaporation is 100 - 97 + 4 + 3 - 2 = 8 mm. The uncorrected level decline is only 3 mm.

This is a balance example, not a measured NiuBoL field result. If the refill is recorded in liters, convert it using the pan's actual surface area; do not insert the volume number into a depth calculation. A rain gauge beside the pan also needs suitable exposure and matching intervals before its catch is treated as the pan's rainfall input.

Match the evaporation instrument to acquisition hardware

The NBL-W-ES evaporation sensor is used with a pan or tank. Its 0-1000 mm range is the sensing range, not a promise that daily evaporation can reach that value or that the logger correctly compensates for every transfer. Keep the instrument specification and the calculation requirements distinct.

Use the weather channels to explain changing evaporation

Wind, air temperature, humidity and solar radiation help describe the conditions over the water surface. Keep their positions and observation intervals with the pan record. A sensor inside a crop canopy or greenhouse describes another exposure and should not silently replace the field weather series when comparing evaporation periods.

Choose the light quantity carefully. Solar irradiance in W/m2, illuminance in lux and a photon-based crop-light measurement are different inputs. The NiuBoL solar radiation sensor can supply a selected broadband observation, but a light field in a dashboard is not enough to establish its quantity or calibration.

Do not equate pan evaporation with an irrigation instruction

The FAO-56 pan evaporation method relates pan evaporation to reference evapotranspiration through a pan coefficient, Kp. That coefficient depends on pan type and environment, including surrounding cover, wind and humidity. It should not be copied to an unidentified tank solely because both results use millimeters.

Reference evapotranspiration is still not a complete field irrigation requirement. Crop stage, root-zone conditions, rainfall contribution and the irrigation arrangement remain relevant. Soil-moisture measurements show conditions around selected probes; the evaporation tank does not measure them. Use the observations together without giving each sensor credit for quantities it cannot directly observe.

Preserve refill and cleaning events in the data

Record the level immediately before and after a measured refill where practical, together with the event time and quantity. If cleaning empties the tank or changes the observation arrangement, mark the affected period. A software filter should not simply remove every upward step, because real rainfall can produce a similar pattern.

Check tank stability, leakage and contamination when an unexplained loss appears. Crop growth or a new shelter can alter exposure through the season. Record those changes before interpreting a lower evaporation total as a regional weather change. Comparisons between two fields also need the pan configuration and surroundings, not just matched sensor model names.

Commission measured level and calculated evaporation separately

Check the level output against an appropriate reference before testing the water-balance calculation. Use an approved measured addition or removal to verify sign, scale and event handling. Then reproduce one complete calculation from exported level, rain and maintenance records. A correct current-loop scale does not validate the logger's evaporation algorithm.

For an RS485 version, obtain the actual protocol and register definitions; do not assume Modbus RTU solely from the electrical interface. For analog output, verify the supplied range and invalid-signal treatment. If any required interval is missing, flag the evaporation result rather than replacing the absent rain or level with zero.

Evaporation Data FAQ

Q1. Why did evaporation appear negative after rain?

Rain can raise the water level while evaporation continues. Review the rainfall input, timing and any refill before interpreting the raw level difference as evaporation.

Q2. Can the logger infer every refill automatically?

Not reliably from a level step alone. A change can also be rain or servicing. Record the measured transfer and event type or mark the affected balance as uncertain.

Q3. Does the temperature range permit winter evaporation measurements in ice?

No such conclusion follows. Instrument operation and the observation method are separate. Apply an appropriate cold-season procedure when the pan no longer contains a normal liquid-water surface.

Q4. Can one Kp value be used for every station?

Not without a justified method. Pan geometry, siting and surrounding conditions affect the relationship. Establish the applicable coefficient and validation rather than assuming universality.

Q5. What should a project quotation define?

Specify the pan and mounting arrangement, sensor output, rain measurement, transfer-event recording and required calculation/export. State whether NiuBoL is supplying sensing hardware only or the complete acquisition configuration.

Summary

Agricultural weather station evaporation data become useful when level changes are reconciled with rain, refills and other water transfers. NiuBoL evaporation and weather instruments can supply the observations. Keep the balance reproducible and distinguish pan loss from reference evapotranspiration, soil moisture and the final irrigation decision.