Pierpaolo Duce

An evaluation of common evapotranspiration equations

Abstract A comparison is made between the Pruitt and Doorenbos version of an hourly Penman-type equation, the Food and Agriculture Organization (FAO) hourly Penman-Monteith equation, and an independent measure of reference evapotranspiration (ET0) from lysimeter data. Reducing the canopy resistance improved the hourly FAO Penman-Monteith estimates. Daytime soil heat flux density is estimated as 10% of net radiation in the FAO hourly Penman-Monteith equation; however, the measured soil heat flux density under grass that was never shorter than 0.10 m in this study was between 3% and 5% of net radiation. The daytime totals of hourly ET0 from the hourly Penman-Monteith and Pruitt-Doorenbos equations and ET0 from the 24-h FAO Penman-Monteith equation were computed using data from five Italian and five Californian stations. A comparison showed that all of the equations gave acceptable results. The Pruitt-Doorenbos equation may slightly over-estimate ET0 in conditions of summertime cold air advection.

Estimating sensible and latent heat flux densities from grapevine canopies using surface renewal

Fine-wire thermocouples were used to measure high-frequency temperature above and within canopies and structure functions were employed to determine temperature ramp characteristics, which were used in a fundamental conservation of energy equation to estimate sensible heat flux density. Earlier experiments over dense, tall, and short canopies demonstrated that the surface renewal method works, but requires a correction for uneven heating (e.g. D0.5 for tall, and D1.0 for short canopies). For sparse canopies, the calibration factor was unknown. Experiments were conducted in grape vineyards in California and Italy to determine whether the surface renewal method works in a sparse canopy and to determine if calibration is necessary. Surface renewal data were collected at several heights in the canopies and these were compared with simultaneous 1-D sonic anemometer measurements. The results indicated that the surface renewal technique provides good estimates of sensible heat flux density under all stability conditions without the need for calibration when the data are measured at about 90% of the canopy height. The values were generally within ca. 45 W m 2 of what was measured with a sonic anemometer. Separating the canopy into two layers provided even more accurate estimates of sensible heat flux density without the need for calibration. The best results were obtained when the lower layer was below the bottom of the vegetation and the upper layer included the vegetation. When combined with energy balance measurements of net radiation and soil heat flux density, using a thermocouple and the surface renewal technique offers an inexpensive alternative for estimating evapotranspiration with good accuracy.

Surface renewal analysis for sensible heat flux density using structure functions

Surface renewal (SR) analysis was used to estimate the sensible heat flux density (H) over different crop canopies (grass, wheat, and sorghum) and the results were compared with eddy covariance measurements. High-frequency temperature traces showed ramp-like structures, and structure functions were used to determine the mean amplitude and duration of these ramps. The ramp characteristics were used to estimate H. A wide range of sensible heat flux density conditions were observed. The accuracy was acceptable, but was dependent on the measurement height, the wind shear at the measurement level, and the time lags used in the structure functions. The use of surface renewal H values in energy balance determination of λE can give results nearly as accurate as those obtained using a sonic anemometer.

Assessing exposure of human and ecological values to wildfire in Sardinia, Italy

We used simulation modelling to analyse spatial variation in wildfire exposure relative to key social and economic features on the island of Sardinia, Italy. Sardinia contains a high density of urban interfaces, recreational values and highly valued agricultural areas that are increasingly being threatened by severe wildfires. Historical fire data and wildfire simulations were used to estimate burn probabilities, flame length and fire size. We examined how these risk factors varied among and within highly valued features located on the island. Estimates of burn probability excluding non-burnable fuels, ranged from 0-1.92 × 10-3, with a mean value of 6.48 × 10-5. Spatial patterns in modelled outputs were strongly related to fuel loadings, although topographic and other influences were apparent. Wide variation was observed among the land parcels for all the key values, providing a quantitative approach to inform wildfire risk management activities. Language: en

Evaluation of FARSITE simulator in Mediterranean maquis

In the last two decades, several models were developed to provide temporal and spatial variations of fire spread and behaviour. The most common models (i.e. BEHAVE and FARSITE) are based on Rothermels original fire spread equation and describe fire spread and behaviour taking into account the influences of fuels, terrain and weather conditions. The use of FARSITE on areas different from those where the simulator was originally developed requires a local calibration to produce reliable results. This is particularly true for Mediterranean ecosystems, where plant communities are characterised by high specific and structural heterogeneity and complexity. To perform FARSITE calibration, an appropriate fuel model or the development of a specific custom fuel model is needed. In this study, FARSITE was employed to simulate three fire events in Mediterranean areas using different fuel models and meteorological input data, and the accuracy of results was analysed. A custom fuel model designed and developed for shrubland vegetation (maquis) provided realistic values of rate of spread, when compared with estimated values obtained using standard fuel models. Our results confirm that the use of both wind field data and appropriate custom fuel models are crucial to obtain reasonable simulations of wildfire events occurring on Mediterranean vegetation during the drought season.

Relationships between seasonal patterns of live fuel moisture and meteorological drought indices for Mediterranean shrubland species

Measurements of seasonal patterns of live fuel moisture content and ignitability (in terms of time to ignition) of four Mediterranean shrub species were performed in North Western Sardinia (Italy). Relationships between the two variables were evaluated. Relationships between live fuel moisture content and environmental conditions (i.e. rainfall, air temperature and soil moisture) were analysed. Two groups of species were identified in relation to the different response of live fuel moisture content to seasonal meteorological conditions. Seasonal patterns of live fuel moisture content were also compared with five meteorological drought indices: Duff Moisture Code and Drought Code of the Canadian Forest Fire Weather Index System, Keetch–Byram Drought Index, Canopy Drought Stress Index and Cumulative Water Balance Index. In addition, the capability of the meteorological drought indices to describe moisture variation for each species was evaluated. Although the Drought Code was formulated to describe changes in the moisture content of dead fuel, it was shown to have a good potential for modelling live fuel moisture variation of a group of shrubland species that are sensitive to meteorological conditions, with a clear and large decrease of moisture content during the drought season.

Seasonal variations of live moisture content and ignitability in shrubs of the Mediterranean basin

The objectives of the present study were (i) to describe the seasonal pattern of ignition delay (ID time) and moisture content of live fine fuel (LFMC) for eight common shrub species of the Western Mediterranean Basin, and (ii) to evaluate the relationships between LFMC and ignitability. The experiment was carried out in a shrubland area located in Sardinia, Italy. LFMC and time to ignition or ID time values were determined monthly or twice a month throughout the year. Ignitability was determined by laboratory tests. Meteorological data were also collected from a weather station located in the study area. Significant linear regression equations of ID time v. LFMC were found for most of the species, with R2 values ranging from 67 to 94%. In general, very low values of ID time were observed when LFMC ranged from 70 to 100%. In addition, two groups of species were identified, depending on seasonal changes of both LFMC and ID time (or ignitability): (i) species with LFMC not greater than 100% and high ignitability values all throughout the year, and (ii) species with large seasonal variability of LFMC and ignitability values that were low in winter and high in summer. Significant differences in ignitability among species were also observed.

Carbon and nitrogen balances for six shrublands across Europe

[1] Shrublands constitute significant and important parts of European landscapes providing a large number of important ecosystem services. Biogeochemical cycles in these ecosystems have gained little attention relative to forests and grassland systems, but data on such cycles are required for developing and testing ecosystem models. As climate change progresses, the potential feedback from terrestrial ecosystems to the atmosphere through changes in carbon stocks, carbon sequestration, and general knowledge on biogeochemical cycles becomes increasingly important. Here we present carbon and nitrogen balances of six shrublands along a climatic gradient across the European continent. The aim of the study was to provide a basis for assessing the range and variability in carbon storage in European shrublands. Across the sites the net carbon storage in the systems ranged from 1,163 g C m � 2 to 18,546 g C m � 2 , and the systems ranged from being net sinks (126 g C m � 2 a � 1 ) to being net sources (� 536 g C m � 2 a � 1 ) of carbon with the largest storage and sink of carbon at wet and cold climatic conditions. The soil carbon store dominates the carbon budget at all sites and in particular at the site with a cold and wet climate where soil C constitutes 95% of the total carbon in the ecosystem. Respiration of carbon from the soil organic matter pool dominated the carbon loss at all sites while carbon loss from aboveground litter decomposition appeared less important. Total belowground carbon allocation was more than 5 times aboveground litterfall carbon which is significantly greater than the factor of 2 reported in a global analysis of forest data. Nitrogen storage was also dominated by the soil pools generally showing small losses except when atmospheric N input was high. The study shows that in the future a climate-driven land cover change between grasslands and shrublands in Europe will likely lead to increased ecosystem C where shrublands are promoted and less where grasses are promoted. However, it also emphasizes that if feedbacks on the global carbon cycle are to be predicted it is critically important to quantify and understand belowground carbon allocation and processes as well as soil carbon pools, particularly on wet organic soils, rather than plant functional change as the soil stores dominate the overall budget and fluxes of carbon.

Multi-GCM projections of future drought and climate variability indicators for the Mediterranean region

Abstract Future climate conditions for the Mediterranean region based on an ensemble of 16 Global Climate Models are expressed and mapped using three approaches, giving special attention to the intermodel uncertainty. (1) The scenarios of mean seasonal temperature and precipitation agree with the projections published previously by other authors. The results show an increase in temperature in all seasons and for all parts of the Mediterranean with good intermodel agreement. Precipitation is projected to decrease in all parts and all seasons (most significantly in summer) except for the northernmost parts in winter. The intermodel agreement for the precipitation changes is lower than for temperature. (2) Changes in drought conditions are represented using the Palmer Drought Severity Index and its intermediate Z-index product. The results indicate a significant decrease in soil moisture in all seasons, with the most significant decrease occurring in summer. The displayed changes exhibit high intermodel agreement. (3) The climate change scenarios are defined in terms of the changes in parameters of the stochastic daily weather generator calibrated with the modeled daily data; the emphasis is put on the parameters, which affect the diurnal and interdiurnal variability in weather series. These scenarios indicate a trend toward more extreme weather in the Mediterranean. Temperature maxima will increase not only because of an overall rise in temperature means, but partly (in some areas) because of increases in temperature variability and daily temperature range. Increased mean daily precipitation sums on wet days occurring in some seasons, and some parts of the Mediterranean may imply higher daily precipitation extremes, and decreased probability of wet day occurrence will imply longer drought spells all across the Mediterranean.

Assessing temporal variation of primary and ecosystem production in two Mediterranean forests using a modified 3-PG model

ContextForest ecosystem carbon uptake is heavily affected by increasing drought in the Mediterranean region.AimsThe objectives of this study were to assess the capacity of a modified 3-PG model to capture temporal variation in gross primary productivity (GPP), and ecosystem net carbon uptake (NEE) in two Mediterranean forest types.MethodsThe model was upgraded from a monthly (3-PG) to a daily time step (3-PGday), and a soil water balance routine was included to better represent soil water availability. The model was evaluated against seasonal GPP and NEE dynamics from eddy covariance measurements.ResultsSimulated and measured soil water content values were congruent throughout the study period for both forest types. 3-PGday effectively described the following: GPP and NEE seasonal patterns; the transition of forest ecosystems from carbon sink to carbon source; however, the model overestimated diurnal ecosystem respiration values and failed to predict ecosystem respiration peaks.ConclusionsThe model served as a rather effective tool to represent seasonal variation in gross primary productivity, and ecosystem net carbon uptake under Mediterranean drought-prone conditions. However, its semi-empirical nature and the simplicity inherent in the original model formulation are obstacles preventing the model working well for short-term daily predictions.