The 321 L/kg Question: How Climate Changes Rice Water Requirement Without Recreating the Water Demand of Flooded Rice

KP-201 compared mature PQNK rice with conventional flooded rice and recorded approximately 321 litres of applied water per kilogram of paddy under PQNK against 4,699 litres per kilogram in the conventional flooded reference, roughly a 7:100 relationship in applied water. A question raised from Saudi Arabia exposed a point that deserves explicit clarification: the 321 L/kg figure and the 7:100 production-system comparison are related, but they are not the same claim. The 7:100 ratio describes the difference between two production systems, flooded and non-flooded. The 321 L/kg figure is the measured PQNK field reference under the environmental conditions represented by that crop, roughly 70% relative humidity, around 5 km/h wind velocity and ambient temperature reaching about 40°C, under which 1,600 kg of paddy per acre was produced with approximately 513,950 litres of applied crop-life water. It should not be treated as a universal physiological constant for rice, and it was derived from field water applied divided by paddy produced, not from an independent measurement of leaf transpiration alone.

Water taken up by rice roots is not incidental to the plant; it carries dissolved mineral nutrients, supports photosynthesis, cell expansion and temperature regulation, and most of it ultimately leaves through the leaves as transpiration, at a rate that responds to the surrounding atmosphere rather than staying fixed. The claim that rice water is lost only through evaporation and not through transpiration is therefore too absolute. PQNK does not eliminate transpiration, which is part of normal plant function; what it eliminates is the avoidable, non-productive water use created by maintaining a flooded field. The amount of water required above the 321 L/kg reference depends principally on atmospheric and soil conditions: higher ambient temperature increases evaporative energy and cooling demand, lower relative humidity increases the drying power of the air, higher wind velocity continually replaces moist air around the leaves with drier air, and stronger solar radiation raises both leaf temperature and evaporative demand. These variables interact rather than acting independently, which is why the paper frames the adjustment through evaporative demand and vapour-pressure deficit rather than a single climatic correction applied everywhere.

Rainfall and plant water requirement are also easily conflated but need to be kept separate. Low rainfall means irrigation must supply a larger share of the crop's water, but it does not by itself mean the plant transpires more; the plant-side requirement is driven by atmospheric demand and crop physiology, while the source side is supplied by rainfall, dew or condensation where meaningful, stored soil moisture, capillary contribution where present, and irrigation. In a dry inland environment such as Saudi Arabia, rainfall's contribution shrinks while hotter, drier air may simultaneously raise the plant's own water demand, two separate effects that should be measured separately rather than assumed to move together. Mulch's dependable role is reframed accordingly: it is not primarily a means of capturing atmospheric humidity, since that contribution can be small in a dry inland climate, but a way of protecting the water already present in the soil, by shielding the surface from direct solar radiation and wind, moderating temperature and reducing non-productive evaporation, a role that holds regardless of ambient humidity.

For preliminary planning in substantially hotter, drier conditions, the paper offers a working allowance of 20% above the 321 L/kg reference, approximately 385 L/kg, explicitly as a planning assumption to be tested by field measurement, not a measured universal correction factor; the field's actual requirement is ultimately governed by Soil Moisture Management, and measured irrigation divided by measured yield at harvest gives the true figure for that environment. The paper also separates the approximately 93% saving from the irrigation method it is compared against: it is relative to a continuously flooded reference, so if rice is already grown with drip, sprinkler or another efficient non-flood method, the percentage saving when compared with PQNK will be smaller, even though the underlying PQNK water reference has not changed, only the denominator the percentage is calculated against. A useful Saudi Arabian trial would therefore compare the same rice variety under conventional flooded production, an efficient non-flood irrigation treatment where practical, and mature PQNK, over the same climatic period and comparable soil conditions, recording applied irrigation, rainfall, relative humidity, temperature, wind, solar conditions, soil moisture at depth, crop development and final yield, so that productive crop-water demand can be separated from the avoidable losses created by irrigation architecture itself.