11:25 AM - 11:40 AM
[ACG08-09] Evaporation from forest during rainfall: a basic principle of moisture transport from the ocean to inland continent
Keywords:Canopy interception, Splash droplet, Biotic pump
Introduction
Evaporation of canopy interception I accounts for some 20% of rainfall. Because of I, evapotranspiration ET from forest is larger than any other surfaces on our planet. However, the amount of I estimated by the heat balance equation sometimes severely underestimates the observed values, which has been an enigma. Murakami1) proposed that I is not evaporation from wet canopy surface but evaporation of splash droplets of raindrops. The objective of the present study is 1) to try to prove splash droplet evaporation (SDE) hypothesis based on measurements, and 2) to combine I with the biotic pump theory2) that presumes precipitation in the inland of a forested continent is driven by ET of forest.
Methods
Artificial Christmas trees were arranged on a tray and were placed outside under the natural rainfall3). Drainage from the tray as net rainfall PN and the weight of a single tree to calculate water storage on canopy S were measured. Gross rainfall PG and PN were measured with a 5-minutes interval and S was a 1-minute interval. Separation time of rainfall Spt that divides rainfall into each individual rain event was set at 6 hours. The storm break time Sbt is defined as an intra-storm separation time and was set at 20 minutes, which divides a rain event into sub-rain events, i.e. 20 minutes ≤ Sbt < 6 hours ≤ Spt. I during Sbt is defined as ISbt, I after rainfall ceases as IAft, and I during rainfall when rainfall is observed as IR. IR and ISbt can be calculated using PG, PN and S, while IAft is derived from S only.
Results and discussion
Figure shows ΣIR, ΣISbt, and IAft against PG on a rain event basis for a Christmas tree stand. IR and ISbt are shown as the sum of the values since the rain event usually consists of plural sub-rain events. For PG > 5 mm IAft ≈ 0.5 mm, while ΣISbt is almost zero. It is clear that ΣIR is proportional to PG. For the largest rain event in Figure (below is called Rain event A) PG, ΣIR, ΣISbt and IAft were 84.9 mm, 16.6 mm, 0.5 mm and 0.4 mm, respectively. The largest sub-rain event in Rain event A recorded during nighttime with PG of 59.6 mm, ΣIR of 11.6 mm and an evaporation rate of 1.91 mm/h. The results strongly suggest that rainfall per se drives evaporation during rainfall, i.e. SDE.
Makarieva et al. (2013)2) showed precipitation does not decline with increasing distance from the coast in the continent over thousands of kilometers, if it is covered with forest, and vice versa. They presume that large ET of forest sucks water vapor from the ocean, which is called “the biotic pump”. They also proposed a principle that condensation of water vapor circulates air due to reduction in volume. Their theory can explain removal of water vapor from the canopy and supply of latent heat for IR. As is well known the cause of large ET in forest is I and SDE is the main mechanism of I. That is to say, SDE is the basic principle of the biotic pump.
References
1) Murakami 2006 J Hydrol 319, 72-82.
2) Makarieva et al. 2013 Theor Appl Climatol 111: 79-96.
3) Murakami and Toba 2013 Hydrol Res Let 7: 91-96.
Evaporation of canopy interception I accounts for some 20% of rainfall. Because of I, evapotranspiration ET from forest is larger than any other surfaces on our planet. However, the amount of I estimated by the heat balance equation sometimes severely underestimates the observed values, which has been an enigma. Murakami1) proposed that I is not evaporation from wet canopy surface but evaporation of splash droplets of raindrops. The objective of the present study is 1) to try to prove splash droplet evaporation (SDE) hypothesis based on measurements, and 2) to combine I with the biotic pump theory2) that presumes precipitation in the inland of a forested continent is driven by ET of forest.
Methods
Artificial Christmas trees were arranged on a tray and were placed outside under the natural rainfall3). Drainage from the tray as net rainfall PN and the weight of a single tree to calculate water storage on canopy S were measured. Gross rainfall PG and PN were measured with a 5-minutes interval and S was a 1-minute interval. Separation time of rainfall Spt that divides rainfall into each individual rain event was set at 6 hours. The storm break time Sbt is defined as an intra-storm separation time and was set at 20 minutes, which divides a rain event into sub-rain events, i.e. 20 minutes ≤ Sbt < 6 hours ≤ Spt. I during Sbt is defined as ISbt, I after rainfall ceases as IAft, and I during rainfall when rainfall is observed as IR. IR and ISbt can be calculated using PG, PN and S, while IAft is derived from S only.
Results and discussion
Figure shows ΣIR, ΣISbt, and IAft against PG on a rain event basis for a Christmas tree stand. IR and ISbt are shown as the sum of the values since the rain event usually consists of plural sub-rain events. For PG > 5 mm IAft ≈ 0.5 mm, while ΣISbt is almost zero. It is clear that ΣIR is proportional to PG. For the largest rain event in Figure (below is called Rain event A) PG, ΣIR, ΣISbt and IAft were 84.9 mm, 16.6 mm, 0.5 mm and 0.4 mm, respectively. The largest sub-rain event in Rain event A recorded during nighttime with PG of 59.6 mm, ΣIR of 11.6 mm and an evaporation rate of 1.91 mm/h. The results strongly suggest that rainfall per se drives evaporation during rainfall, i.e. SDE.
Makarieva et al. (2013)2) showed precipitation does not decline with increasing distance from the coast in the continent over thousands of kilometers, if it is covered with forest, and vice versa. They presume that large ET of forest sucks water vapor from the ocean, which is called “the biotic pump”. They also proposed a principle that condensation of water vapor circulates air due to reduction in volume. Their theory can explain removal of water vapor from the canopy and supply of latent heat for IR. As is well known the cause of large ET in forest is I and SDE is the main mechanism of I. That is to say, SDE is the basic principle of the biotic pump.
References
1) Murakami 2006 J Hydrol 319, 72-82.
2) Makarieva et al. 2013 Theor Appl Climatol 111: 79-96.
3) Murakami and Toba 2013 Hydrol Res Let 7: 91-96.