Expert: Heatwaves cause water evaporation from rivers up to three times greater

The heat causes water evaporation from rivers up to three times greater. It is possible that the Vistula will reach a very low level in some places - Prof. Jerzy Kozyra from the Institute of Cultivation, Fertilization and Soil Science in Puławy told PAP. In his opinion, the elimination of hydrological drought will only be possible in the autumn.

The head of the Department of Bioeconomy and Agrometeorology at the Institute of Geology and Natural Resources, Prof. Jerzy Kozyra, pointed out that low water levels in rivers have been observed in Poland for several years now. "This year, it is particularly visible due to low rainfall in autumn and winter. It is good that after an exceptionally warm January, February and May were cool, because there would be even less water in the Vistula now," the expert assessed.

Referring to the hydrological cycle, he recalled that in Poland water management depends on atmospheric precipitation - water first percolates through the soil, penetrates its deeper layers, if there is enough of it (i.e. it exceeds the water capacity of the soil) it reaches the groundwater level and then appears in springs.

"This year, our models describing the amount of water in the soil show that we still have a large water deficit in the deeper layers of the soil. We are trying to include these models in the drought monitoring system, e.g. in the Clim4Cast project of several Central and Eastern European countries, to improve monitoring," he added.

Prof. Kozyra pointed out that agricultural drought can always be observed the fastest by observing the effects on cultivated fields, because agricultural crops react directly to rainfall deficit and evaporation. This, in turn, translates very quickly - as he explained - into weakened plant condition, reduced yields, and consequently financial losses for farmers.

The IUNG expert reported that the largest number of areas where crop losses due to drought will occur this year are in the Lubuskie and Zachodniopomorskie provinces. The central areas of the Łódź and Kuyavian-Pomeranian provinces are also at risk. "Weakened plants are more sensitive to, for example, high temperatures and strong winds. In addition, when it is hot, there is greater pressure from pests and diseases, which may have a negative effect," the agronomist added.

Trees, on the other hand, as he explained, absorb water primarily from deeper soil layers, hence trees in poorer condition may be the first indicator of hydrological drought.

Referring to the recent hot days, he informed that the heat causes water evaporation from rivers up to three times greater. "Normally, this is about 2-3 liters per square meter in the form of evaporation from the surface of, for example, a lake or a pond, while during Thursday's heat - according to evaporation measurements at the IUNG-PIB agrometeorological station in Osiny near Puławy - according to the evaporometer installed at this station, 7 liters of water evaporated from a square meter of water surface," the professor informed.

He added that during high heat - as measurements show - a lot of water evaporates directly from water surfaces, from the soil, but also from plants, as well as from water reservoirs and rivers - which is why less water gets into the rivers.

According to the expert, occasional rains – after such a long water deficit in the soil – will not eliminate the hydrological drought because they are then retained in the surface layers of the soil and do not reach deeper.

"This requires time and long-term precipitation. Most of the precipitation from violent storms does not penetrate into deeper layers of soil, the water does not reach the groundwater level and does not supply springs, this precipitation rather causes surface runoff and, as a consequence, rapid river floods, but such floods are also followed by rapid drops in the water level in these rivers," he emphasized.

When asked how to counteract this, he replied that water must be retained by all possible means on agricultural fields and in drainage ditches, slowing down its flow into rivers.

In order to avoid hydrological drought – as assessed by the professor – about two months of a period with precipitation significantly exceeding multi-year norms are needed. In his opinion, eliminating hydrological drought will most likely be possible only in autumn, when the temperature drops, evaporation will be lower, and longer periods with atmospheric precipitation will appear.

In his opinion, the situation on the Vistula will not change in the near future, because in the summer we have a lot of evaporation due to high temperatures. "It is possible that the Vistula will reach a very low water level in places, which may consequently cause a threat to fish and other aquatic organisms," he said.

He added that the low water levels in rivers could have consequences for industry, including power plants and other large factories that need water for their operations. Ordinary residents will have to reckon with a ban on watering gardens, lawns or filling home swimming pools.

Gabriela Bogaczyk (PAP)

gab/ agz/ jpn/