Impatti, adattamento e vulnerabilita Le cause e le responsabilita dei cambiamenti climatici sono state trattate sul numero di ottobre della rivista Cda. Approfondiamo l’argomento presentando il documento: “Cambiamenti climatici 2007: impatti, adattamento e vulnerabilita” votato ad aprile 2007 dal secondo gruppo di lavoro del Comitato Intergovernativo sui Cambiamenti Climatici (Intergovernmental Panel on Climate Change). Si tratta del secondo di tre documenti che compongono il quarto rapporto sui cambiamenti climatici.

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Climate Change 2007: Impacts, Adaptation and Vulnerability.

Investigating soil moisture-climate interactions in a changing climate: A review

The Intergovernmental Panel on Climate Change (IPCC) Technical Paper Climate Change and Water draws together and evaluates the information in IPCC Assessment and Special Reports concerning the impacts of climate change on hydrological processes and regimes, and on freshwater resources – their availability, quality, use and management. It takes into account current and projected regional key vulnerabilities, prospects for adaptation, and the relationships between climate change mitigation and water. Its objectives are:

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Climate change and water.

It is now well established that forested catchments have higher evapotranspiration than grassed catchments. Thus land use management and rehabilitation strategies will have an impact on catchment water balance and hence water yield and groundwater recharge. The key controls on evapotranspiration are rainfall interception, net radiation, advection, turbulent transport, leaf area, and plant-available water capacity. The relative importance of these factors depends on climate, soil, and vegetation conditions. Results from over 250 catchments worldwide show that for a given forest cover, there is a good relationship between long-term average evapotranspiration and rainfall. From these observations and on the basis of previous theoretical work a simple two-parameter model was developed that relates mean annual evapotranspiration to rainfall, potential evapotranspiration, and plant-available water capacity. The mean absolute error between modeled and measured evapotranspiration was 42 mm or 6.0%; the least squares line through the origin had as lope of 1.00 and a correlation coefficient of 0.96. The model showed potential for a variety of applications including water yield modeling and recharge estimation. The model is a practical tool that can be readily used for assessing the long-term average effect of vegetation changes on catchment evapotranspiration and is scientifically justifiable.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000WR900325

Response of mean annual evapotranspiration to vegetation changes at catchment scale

Terrestrial net primary production (NPP) quantifies the amount of atmospheric carbon fixed by plants and accumulated as biomass. Previous studies have shown that climate constraints were relaxing with increasing temperature and solar radiation, allowing an upward trend in NPP from 1982 through 1999. The past decade (2000 to 2009) has been the warmest since instrumental measurements began, which could imply continued increases in NPP; however, our estimates suggest a reduction in the global NPP of 0.55 petagrams of carbon. Large-scale droughts have reduced regional NPP, and a drying trend in the Southern Hemisphere has decreased NPP in that area, counteracting the increased NPP over the Northern Hemisphere. A continued decline in NPP would not only weaken the terrestrial carbon sink, but it would also intensify future competition between food demand and proposed biofuel production.

Drought-Induced Reduction in Global Terrestrial Net Primary Production from 2000 Through 2009

In order to assess the effects of calibration data series length on the performance and optimal parameter values of a hydrological model in ungauged or data-limited catchments (data are non-continuous and fragmental in some catchments), we used non-continuous calibration periods for more independent streamflow data for SIMHYD (simple hydrology) model calibration. Nash-Sutcliffe efficiency and percentage water balance error were used as performance measures. The particle swarm optimization (PSO) method was used to calibrate the rainfall-runoff models. Different lengths of data series ranging from one year to ten years, randomly sampled, were used to study the impact of calibration data series length. Fifty-five relatively unimpaired catchments located all over Australia with daily precipitation, potential evapotranspiration, and streamflow data were tested to obtain more general conclusions. The results show that longer calibration data series do not necessarily result in better model performance. In general, eight years of data are sufficient to obtain steady estimates of model performance and parameters for the SIMHYD model. It is also shown that most humid catchments require fewer calibration data to obtain a good performance and stable parameter values. The model performs better in humid and semi-humid catchments than in arid catchments. Our results may have useful and interesting implications for the efficiency of using limited observation data for hydrological model calibration in different climates.

Effect of calibration data series length on performance and optimal parameters of hydrological model

. Realistic projections of changes to daily rainfall
frequency and magnitude, at catchment scales, are required to assess the
potential impacts of climate change on regional water supply. We show that
quantile–quantile mapping (QQM) bias-corrected daily rainfall from
dynamically downscaled WRF simulations of current climate produce biased
hydrological simulations, in a case study for the state of Victoria,
Australia (237 629 km 2 ). While the QQM bias correction can remove bias
in daily rainfall distributions at each 10 km  ×  10 km grid point across
Victoria, the GR4J rainfall–runoff model underestimates runoff when driven
with QQM bias-corrected daily rainfall. We compare simulated runoff
differences using bias-corrected and empirically scaled rainfall for several
key water supply catchments across Victoria and discuss the implications for
confidence in the magnitude of projected changes for mid-century. Our
results highlight the imperative for methods that can correct for temporal
and spatial biases in dynamically downscaled daily rainfall if they are to
be suitable for hydrological projection.

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Impact of downscaled rainfall biases on projected runoff changes

Center for Agricultural Water Research in China, China Agricultural University, Beijing, China State Key Laboratory of Urban and Regional Ecology, Research Center for Eco‐ Environmental Sciences, Chinese Academy of Sciences, Beijing, China CSIRO Land and Water Flagship, Canberra, Australia Forestry College of Inner Mongolia Agricultural University, Huhehaote, China Key Laboratory of Agricultural Water Resources, Center for Agricultural Resources Research, Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, Shijiazhuang, China Key Laboratory for Urban Habitat Environment Science and Technology, School of Environment and Energy, Peking University, Shenzhen, China Correspondence Yi He Lü, State Key Laboratory of Urban and Regional Ecology, Research Center for Eco‐ Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China. Email: lyh@rcees.ac.cn Funding information National Natural Science Foundation of China, Grant/Award Number: 41601015. Ministry of Science and Technology of China, Grant/ Award Number: 2012BAC08B01.

Challenge of vegetation greening on water resources sustainability: Insights from a modeling-based analysis in Northwest China

Studies in catchment hydrology: the basics of recharge and discharge.

Changes in climatic characteristics such as seasonal and inter-annual variability may affect ecosystem structure and function, hence alter carbon and water budgets of ecosystems. Studies of modelling combined with field experiments can provide essential information to investigate interactions between carbon and water cycles and climate. Here we present a first attempt to investigate the long-term climate controls on seasonal patterns and inter-annual variations in water and carbon exchanges in an arid-zone savanna-woodland ecosystem using a detailed mechanistic soil–plant–atmosphere model (SPA), driven by leaf area index (LAI) simulated by an ecohydrological model (WAVES) and observed climate data during 1981–2012. The SPA was tested against almost 3 years of eddy covariance flux measurements in terms of gross primary productivity (GPP) and evapotranspiration (ET). The model was able to explain 80 and 71% of the variability of observed daily GPP and ET, respectively. Long-term simulations showed that carbon accumulation rates and ET ranged from 20.6 g C m−2 mon−1 in the late dry season to 45.8 g C m−2 mon−1 in the late wet season, respectively, primarily driven by seasonal variations in LAI and soil moisture. Large climate variations resulted in large seasonal variation in ecosystem water-use efficiency (eWUE). Simulated annual GPP varied between 146.4 and 604.7 g C m−2 y−1. Variations in annual ET coincided with that of GPP, ranging from 110.2 to 625.8 mm y−1. Annual variations in GPP and ET were driven by the annual variations in precipitation and vapour pressure deficit (VPD) but not temperature. The linear coupling of simulated annual GPP and ET resulted in eWUE having relatively small year-to-year variation.

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Lu Zhang

Papers

Effect of calibration data series length on performance and optimal parameters of hydrological model

Impact of downscaled rainfall biases on projected runoff changes

Challenge of vegetation greening on water resources sustainability: Insights from a modeling-based analysis in Northwest China

Studies in catchment hydrology: the basics of recharge and discharge.

Modelling Seasonal and Inter-annual Variations in Carbon and Water Fluxes in an Arid-Zone Acacia Savanna Woodland, 1981–2012