INTRODUCING CLIMATE CHANGE

IN RIVER BASIN MANAGEMENTU. Fratino (1), A. R. Di Santo (2), C. Campana (2), V. Iacobellis (1), L. Romano (2)

(1)Dip. di Ingegneria delle Acque e di Chimica , Politecnico di Bari, 70125 Bari, Italy

(2)Autorità di Bacino della Puglia, 70010 Valenzano (Bari), Italy

Corr. to u.fratino@poliba.it

Castellaneta Marina (TA), 6-9 June 2011Castellaneta Marina (TA), 6-9 June 2011

3rd International Meeting on Meteorology 3rd International Meeting on Meteorology

and Climatology of the Mediterraneanand Climatology of the Mediterranean

ADELFIA rain-gauge

MERCADANTE rain-gauge

Lam

a

Badess

aLam

a

Baro

nale

Figure 1. Picone watershed

Figure 4. Lama Badessa: flow duration curve

Figure 5. Lama Baronale: flow duration curve

Figure 7. Increase of flow velocity (left) and water depth (right) in the climate change scenario, compare to error induced by friction factor estimation

Figure 2. Rainfall-depth-duration curves at Adelfia rain-gauge

Figure 3. Rainfall-depth-duration curvesat Mercadante rain-gauge

+ 5% RUNOFF COEFFICIENT

ABSTRACT

Within an increasing sensibility of international scientific community and public opinion about the issue, the recent European Directives in water policy, the Water Framework Directive 2000/60/EC and the Floods Directive 2007/60/EC - that provide legal instruments for protecting and restoring the water environment, as well as for reducing risks to human health, cultural heritage and economic activity - require the States Member to take into account the eventual trend induced by climate change, from which major changes in yearly and seasonal precipitation and water flow, flooding, coastal erosion and water quality arise.

The effects of the climate change on the hydrological cycle are usually carried out by means of climatic models working on a planetary scale and most of all forecast an drying trend in Southern Europe, unlike a wetting trend in the Northern Europe. On the other hand, at regional scale, the expected changes can be rather different, indeed forecast of precipitation and flow changes at river basin scale are less certain, due to large natural variability in these quantities, as well as the limitation of climate models, and assumptions used to downscale information from climate to hydrological models .

It follows that an important role of water managers would be to adopt plans to climate change impacts, individuating methodologies for evaluation of climate variability scenarios in the basins management, both for water scarcity management and for flood risk management in order to apply efficiently long term measures.

Figure 8. Flooded area for 200-years return time with 21% flow rate increase

Figure 6. Manning's n Roughness Coefficient

INTRODUCTIONEU water policy and above all the Water Framework Directive (WFD) and the Floods Directive (FD), provide legal instruments for protecting and restoring water environment, as well as for reducing risks to human health, cultural heritage and economic activities.In the context of the WFD Common Implementation Strategy, EU Commission planned a research activity on the way to take into account the effects on the hydrological cycle induced by climate change. This research work has the goal to produce guidance for Member States that have to face up to the implementation of EU water policy, taking into account that the climate change is more and more considered to be responsible for the major changes in yearly and seasonal precipitation and consequently water flow, flooding, coastal erosion and water quality.The aforementioned research were summarized by the "Guidance Document n. 24: RIVER BASIN MANAGEMENT IN A CHANGING CLIMATE" (EC 2009), that was largely discussed with a stakeholders and scientists, so to support the water managers in incorporating climate change in the River Basin Management Plans (RBMPs). In assessing climate change, it should be kept in mind that global models are based on several assumptions, so that their results bear a degree of uncertainty. In addition, the coarse spatial resolution of global models is insufficient for application in impact models and thereby for determining the effects of regional and local climate change. To overcome this limitation, regionalization procedures have to be applied and the forecast on global scale downscaled to smaller grid elements. Quantitative forecasts of changes in precipitation and river flows at the river-basin scale remain, however, uncertain, due to the limitations of climate models, as well as scaling issues between climate and hydrological models, even if correction methods have been developed to bring the models closer to a realistic simulation of, for instance, the physical behavior of rainfall-runoff process at the scale of river basins and small catchments. An example is the Continuous Estimation of River Flows (CERF) model, which is a regionalized rainfall-runoff model developed by the Environment Agency of England and Wales and the Centre for Ecology and Hydrology. The study is the first to use catchment-level models for predicting river flows across the whole of England and Wales. The global result is that total annual river flow could drop by as much as 10–15% by the 2050s with a possible decrease in mean monthly river flows during the summer and autumn months of around 50% and a corresponding increase in mean monthly river flows during the winter months of up to 15%.Although no significant general climate-related trend in extreme high river flows that induce floods has yet been detected, there seems to have been an upward trend in flood occurrence in at least some European rivers in the recent past (EC 2009). In general, the upward trend in flood occurrence cannot be unambiguously related to climatic changes, also because long-term trends in hydrological variables are affected by other factors such as land-use change and land management practices. Despite these uncertainties, for countries such as Sweden, Finland and UK, where more in-depth vulnerability studies on climate change impacts on flood risk have been carried out, we can assess that although the information is uncertain, the procedure is able to guarantee that the adaptation measures can already be started.

THE CASE STUDYIn order to evaluate the potential effects of climate change in semi-arid and small basins, to be encompassed in basin management plans, a case study in Southern Italy was carried out.The studied area is the watershed of Picone river (figure 1), whose area is about 270 km2. The watershed is highly permeable and characterized by a large river network with several streams called ‘lame’. These streams presents an ephemeral hydrologic regime, with zero flow for time windows of decades, while peak flow of several hundreds cubic meters per second sometimes occurs.The two main rivers involving the watershed are called Lama Badessa and Lama Baronale, that join into Picone river, just upstream the city of Bari, that in the past was involved in several floods (Mossa, 2007). Because of the uncertain connected to the scaling issue between climate and hydrological models and between global models to local, for investigating the effects at local scale of the global climate change, rainfall data from Mercadante and Adelfia rain-gauges were collected and rainfall-depth-duration curves calculated (figures 2 and 3). Supposing a rainfall duration equal to the watershed concentration time an increment of about 11% for 30-years return time, of about 15% for 200-years and of 17% for 500-years was found. Considering at the same time the change of the land use in the watershed in the last decades equal to 5% of the runoff coefficient, a peak flow increase equal to 16% for 30-years return time, 21% for 200-years and 23% for 500-years can be estimated. For evaluating the effects of flow rate increase for the basin management plan, a two-dimensional scheme was used for defining the change of the flooded area extension and for estimating water depth and flow velocity increase. The input data are the Lama Badessa (figure 4) and Lama Baronale (figure 5) flow duration curve in the actual scenario and in case of climate change.Due to the characteristics of the investigated area, there is not significant increase of flooded area in case of the climate change scenario (figure 8). The water depth increment percentage due to different flow rate increases in the climate change scenario (figure 7 - dark blue is with an increment of 16% and light blue is with an increment of 21%) is less than 10% on average. An important data grows out of the comparison to the error induced by the friction factor estimation (figure 6) that is greater than 15%. For the flow velocity the increment is on average of 5% and less than the error induced by the roughness on the velocity estimation (figure 7).

CONCLUSIONDespite the uncertainties of the scaling methodologies of the global models, adaptation in the flood risk management to the climate change need to be started. In this preliminary stage of the Floods Directive 2007/60/CE implementation, it has been analyzed a watershed in South Italy, with a drainage area of about 270 km2 and characterized by ephemeral hydrologic regime. Although the climate change is in progress, the historical series of the pluviograph stations show not always a positive trend of rainfall depth in the last decades, confirming the limitations of climate models to reproduce the physical behavior of small catchments. Assuming the increase of rainfall-depth-duration curves of the upstream pluviograph station equal to 11% for 30-years return time, 15% for 200-years and 17% for 500-years and contextually the change in land use of the catchment (that determines 5% increase of runoff coefficient), the hydrographs has been estimated in the climate change scenario: the increment of peak flow rate is 16% for 30-years return time, 21% for 200-years and 23% for 500- years. The comparison of the hydraulic simulations in the area strictly related to the outlet of the Picone watershed in the climate change scenario shows that there are not significant modification of flooded area (with the exception of floods with a low probability) and that the increment of the water depth and of flow velocity (respectively 10% and 5%) is less than the error induced for example by roughness estimation. In conclusion, for the case examined, it seems that climate change have influences in the hydraulic risk map less than the uncertainties related to the estimation of hydraulic and hydrologic parameters, such as runoff coefficient, roughness or even topographic information.

References Mossa M. (2007), The floods in Bari: What history should have taught - Journal of Hydraulic Research Vol. 45, No. 5 (2007), pp. 579–594European Communities (2009), River basin management in a changing climate – a Guidance document No. 24. pp. 134Chow V.T. (1959), Open Channel Hydraulics, McGraw-Hill, USA , ISBN 07-010776-9Arcment G.J.Jr. & Schneider V.R. (1989), Guide for selecting Manning’s roughness coefficients for natural channel and floodplains, United States Geological Survey, Water-Supply Paper 2339