Soil water regime and evapotranspiration of sites with trees and lawn in Moscow

Keywords: Urban vegetation, Tilia cordata, linden, lawn, grass, Leaf Area Index, LAI, digital image processing, evapotranspiration, water stress, electric conductivity, salinity stress, Makkink’s radiation model, deep percolation, water infiltration, runoff, modelling Situations where tree groups of the species Tilia cordata grow together with lawn grass (trees overlapping grass) were studied on five locations in Moscow, Russia, during six periods of the growing season of 2004. The measurements included: detailed descriptions of the soil profiles, tree and lawn dimensions, and, for each period, leaf area index (LAI), soil water content, and soil electric conductivity (EC). LAI was determined through taking photos with a digital camera and processing the photos with a digital image processing program. Using weather and LAI data and vegetation dimensions, the values of potential evapotranspiration of the vegetation combinations were calculated. These calculations followed FAO guidelines for computing crop water requirements. The reference evapotranspiration was also calculated according to Makkink’s radiation model. The results resembled the values of the FAO reference. The measured values of soil water content were used to identify sites and periods with reduced evapotranspiration due to water stress. It appeared that incidence of water stress was very common. The measured soil water content values were transformed into ratios of actual evapotranspiration and potential evapotranspiration: so-called water stress factors. Using these factors, the actual evapotranspiration was calculated from the potential evapotranspiration values. The water regimes of each object and period were analysed. Deep percolation occurred in early spring and late autumn. The possibilities for rainwater to infiltrate the soil were very limited, due to degeneration of soil structure. The water balance of the root zones indicated that the root-zone volumes were smaller than in average forest conditions, and that runoff was extremely high.