Reference evapotranspiration sensitivity to climatic variables in Genoa, Italy. 

  Reference evapotranspiration sensitivity to climatic variables in Genoa, Italy. Komal*, A. Palla Department of Civil Chemical and Environmental Engineering, University of Genova, Montallegro 1, Genoa, Italy*Corresponding author email: komal.jabeen@edu.unige.itTo develop nature-based solutions for sustainable urban water management, it is important to understand the effect of climatic variables over Reference Evapotranspiration (ET0). The aim of this study is to observe the effect of variation in independent meteorological variables in estimation of ET0 through Penman Montieth Equation for a green roof experimental site located in Genoa, Italy. Sensitivity of ET0 along the year is investigated by using relative changes in the dependent variable (ET0 in mm/day) against relative changes in the following independent variables: maximum temperature (Tmax in °C), solar radiation (Rs in MJ/m²/day), wind velocity (U in m/s), maximum relative humidity (RHmax in %) while keeping all other dependent variables constant. Reference evapotranspiration at daily scale was assessed by the Penman–Monteith equation according to the guidelines given in FAO irrigation and drainage paper 56.     A dimensionless sensitivity coefficient, SCX for each independent variable, X, is defined as follows:where the subscripts Base refers to the observed set of dependent variables while the subscript X refers to the subset including the modified dependent variable. The modified variables are built according to a matrix of 40 values characterized by a 5% increase.In the daily analysis, the SC average values for T, Rs and U are positive while for RHmax is negative. According to the results, FAO-56 is highly sensitive to the value of Rs as confirmed by the average value of SC equal to 0.3147) that means every 5% increase in Rs would result in 31.47% rise in the rate of ET0 on average annually. Average value of SC for Tmax was 0.1641, for RH max was -0.0951 and for U was found to be 0.2063. SC values were ranked as Rs > U > T max > RHmax but this impact of increase or decrease does not work identically all through the year since all the SC coefficients show seasonality. In particular, for summer season the sensitivity of ET0 is higher for Rs and Tmax as the intensity and duration of incoming solar radiation is high while the effect of windspeed is negligible in summer, contrary in winter season, ET0 is negatively affected by Rs and Tmax.  follows the pattern of  but it varies in magnitude. The effect of wind is quite noticeable in winter when windspeed is generally higher in Genoa with respect to temperature and solar radiation becoming the ET0 driving factor.  As Genoa is a coastal city and the climate is mainly humid, the effect of maximum relative humidity is almost constant all through the year. This study can serve as a baseline to understand the sensitivity of FAO Penman equation across different urban zones of Italy and consequently to understand the hydrologic performance nature-based solution under climate change.