Isotopic estimation of the evapo-transpiration flux in a plain agricultural region (Po plain, Northern Italy)

Abstract

Samples of water vapour and monthly precipitation were collected in Pavia, located 50km south of Milan (Western Po plain, Northern Italy), over a period of 19months, from March 2006 to September 2007. Results are interpreted in relation to the local climatic factors (temperature and precipitation rates), and to air mass circulation patterns, derived from sea level pressure maps, geopotential maps and satellite images. Since most water vapour samples represent a mixture of continental air masses and local evapo-transpiration fluxes, a computational method based on the stable isotope content (EMMA) has been used to evaluate the percentage of the different components and to quantify the local vapour fraction. The regression line equation for rainwater samples is: δ2Hvs.VSMOW=8.8(±0.5)·δ18Ovs.SMOW+14.5(±3.5)‰(R2=0.96;n=17)The slope of the line is extremely high and probably related to the dataset used, which includes two summer seasons and one winter season. In addition, the latter was somewhat anomalous, with recorded average temperatures higher than the average calculated for the years 1970–2002. The mean annual weighted δ18O in rainwater samples is equal to −6.90±2.2‰. The regression line equation for water vapour samples is: δ2Hvs.VSMOW=6.8(±0.3)·δ18Ovs.SMOW-7.4(±4.9)‰(R2=0.92;n=37).The two regression lines meet at δ18O=−10.82±13.97‰. This value appears more depleted than the mean annual weighted precipitation value, but is close to the isotope composition of the phreatic aquifer (δ18O=−9.0±0.5 to −10.4±0.3‰). In addition, the slope of the water vapour regression line (6.8) indicates evaporation under high relative humidity (Rh=95%).The isotope composition of the water vapour in the Pavia area results from three main components: moisture carried by continental cold circulation, by maritime (Atlantic and Mediterranean) circulations and by the local evapo-transpiration flux. The latter is more intense in late spring and summer, due the maximum vegetation activity but also to irrigation and rice field flooding, and to the consequent maximum rise of the water table level. The turbulence due to the dynamic nature of the troposphere mixes local atmospheric moisture with water vapour carried by advection from other regions. Even in the most obvious situations of no circulation, phenomena of exclusive local evapo-transpiration are seldom observed. Circulation of air masses from the Mediterranean and the Atlantic carry the major part of atmospheric moisture towards the Po plain, while only cold and dry air masses from the NE, which are not found in the summer, are able to completely lower the local isotopic signal of the vapour, because of the substitution effect caused by their higher density. The expected correlation between the evapo-transpiration flux intensity and the precipitation amount is significant, and little difference in the local evapo-transpiration flux is observed between 2006 and 2007. On the other hand, the isotope composition of water vapour testifies to the importance of irrigation as a source of local vapour, evidencing, even at mid-latitudes, a regional scale feedback between land use and climate.