HYDROLOGICAL MODELLING OF THE CAÑETE RIVER BASIN AND ASSESSMENT OF CLIMATE CHANGE IMPACTS 1

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1 HYDROLOGICAL MODELLING OF THE CAÑETE RIVER BASIN AND ASSESSMENT OF CLIMATE CHANGE IMPACTS 1 High mountain glacier melting and shrinkage is a process that is aggravated by the effects of climate change (WGMS, 2012; Gardner, 2013). The reduction in ice coverage and the emergence of new bodies of water as a result of the aforementioned process can have serious repercussions on the local socioeconomic and ecosystemic system (Bradley et al., 2006; Drenkhan et al., 2015).Context transformation will be reflected in a critical relationship between water supply and demand, particularly in regions where there is (seasonal) water shortage due to current hydroclimatic conditions. The Glaciares+ project seeks to have a hydrological analysis tool to identify current and future water reserves as well as potential hazards at basin level. A first study and development of the tool was assessed for the Cañete River Basin, where no such research had yet been carried out. The results of the study are summarized in this brief. The aim of the study was to: 1) build and calibrate a hydrological model that enables the assessment of the current water availability (in both glacier and non-glacier areas), and 2) analyze the impact of climate change under certain scenarios. CAÑETE RIVER BASIN AND THE HYDROLOGICAL MODEL The Cañete River Basin is located in the provinces of Cañete and Yauyos, Lima region, Peru. It has a total surface of km 2, of which only 15 km 2 (approximately 0.24%) is glacier. This area consists of small glaciers at high altitude, above m asl. In terms of precipitation, the Cañete River Basin has regions with very dry conditions (the lower section) and those with humid conditions (the upper section). The hydrological model 2 of the basin was developed with RS MINERVE software which enables simulations of formation and transport of flows through hydraulic and hydrological modelling using a semi-distributed conceptual framework 3. RS MINERVE takes into account certain hydraulic elements (e.g. dams, floodgates and spillways) as well as specific hydrological processes in the glacier and non/glacier areas (e.g. snowmelt, surface flow and groundwater flow resulting from infiltration). In order to prepare the model, available geographical information (Digital Terrain Model with 90 m resolution) was combined with hydraulic elements (that have a big influence in managing flows of the Cañete River). The Paucarcocha and Capilluca dams in the upper and middle sections of the basin respectively were also taken into account. For an efficient use of the model, and considering the orography and presence of hydraulic elements, the basin was divided into 26 sub-basins. 1 This executive brief has been drawn up based on the report Modelización Hidrológica de la cuenca del Cañete y evaluación del impacto del cambio climático, developed by J. Fluixá Sanmartín, J. García Hernández, C. Huggel, H. Frey, R. Muñoz, L. Barriga, M. Cerna and J. Núñez. 2 A hydrological model is a representation of reality that enables simulation of rain-runoff processes in a basin with varying degrees of complexity, from simple equations to the use of powerful calculation softwares. The model makes it possible to understand the hydrological processes and contributions from glacier and non-glacier areas, to assess current water availability and to foresee possible basin contributions in the long term according to future climate conditions. 3 In this type of model the basin is subdivided into several units or sub-basins, and one of the available models is used in each unit. One of the advantages of a semi-distributed conceptual model is that it takes into account the heterogeneity of the sub-basins, enabling greater precision in modelling hydrological processes. It also requires less soil morphological information, making calculations quicker (calibration, validation, simulation, etc.).

2 Each sub-basin was in turn subdivided into 400 m altitude bands from which dependent processes were evaluated (e.g. temperature variation and evapotranspiration). A total of 116 altitude bands were generated and used as the basis for the RS MINERVE model. The process was carried out using two hydrological models: 1) GSM (Glacier Snow Melt) to model the glacier area, and 2) SOCONT (Soil CONTribution) to model the non-glacier area of the basin. In order to facilitate the management of the large number of hydrological elements created (14 glacier elements and 102 non-glacier elements), the objects from both models were grouped into 34 sub-models. The remaining hydraulic elements were created and added based on this division. Finally, all the elements were connected in the direction of flow down to the basin outlet in the Pacific Ocean. The calibration of the model (that is, identifying the values of the parameters for which the series of simulated data has the closest fit to observed data) was based on: Figure 1. Hydrological model superimposed on the study basin. Meteorological data from the stations closest to the basin. These stations are managed by the National Meteorology and Hydrology Service (SENAMHI). The model required reliable precipitation and temperature data. This requirement led to selecting 11 complete series of data for precipitation and 6 for temperature to be used in the calibration process. Hydrological data of discharge observed at different points in the basin. These were gathered, processed and supplied by CELEPSA/Ambiand. The reference period used to calibrate the model and evaluate the water resources is the period for which there is a greater amount of reliable hydrometeorological data. After analyzing the data series, it was decided that would be used as the reference period. The model was calibrated with the basin divided into zones (upper, middle and lower) which were further subdivided into subzones. Each subzone was calibrated independently with the nearest gauging station downstream. Due to the short length of the flow series gauged, it was only possible to use a single calibration phase, without the subsequent validation. The quality of the resulting model was evaluated using efficiency indicators (e.g. Nash Efficiency or Kling-Gupta Efficiency), which estimate how close the simulated flows are to those observed. The greater the similarity, the higher the value of the indicator. Despite the limited amount of available data, the hydrological model built for the Cañete River Basin offers adequate results with the exception of a couple of zones where the calibration did not yield optimal quality, primarily due to the lack of hydrological records. Once the calibration of subzones was complete, the values of the parameters measured were included in the complete model of the Cañete River Basin. For the period of reference, the results show the small contribution of the glacier parts to the total basin water contribution as a result of their limited presence in the territory (15 km2 of glacier surface in comparison to km2 of non-glacier surface). This coincides with the average contributions

3 of the sub-basin entering Capilluca dam (where the non-glacier parts contribute between 97% and 100% of the water reserve). However, in the dry season (June to September) there is a significant contribution from glacier and snow melt from the upper part of the basin that provides directly to the Paucarcocha dam (figure 2). This aspect is significant since the Paucarcocha dam (109 hm 3 capacity) makes a significant contribution to water regulation in the Cañete River Basin. Percentages of average monthly contribution Paucarcocha 100% Percentage of total average monthly contribution 90% 80% 60% 50% 40% 30% 20% 10% 0 January February March April May June July August September October November December Glacier area Non-glacier area Figure 2. Percentages of average monthly contribution of the glacier and non-glacier parts to the Paucarcocha dam. INCORPORATION OF NEW CLIMATE CONDITIONS Incorporating future meteorological and glacier conditions into the hydrological model for the medium (mid-21 st century) and long term (end of the 21 st century) involved reviewing the following technical-scientific documents: Cañete: Escenarios de cambio climático (Meteodat, 2016). This study uses climate projections to obtain changes in future meteorological variables, particularly precipitation and temperature. It proposes the use of two climate models (GISS- E2-H_p3 and MPI-ESM-MR) under two emission scenarios (RCP2.6, optimistic; and RCP8.5, pessimistic). Report El futuro del clima y de los glaciares en el Perú (Schauwecker, 2016) and scientific article The freezing level in the tropical Andes, Peru: An indicator for present and future glacier extents (Schauwecker et al., 2017). Both works investigate the expected changes on glacier surfaces in Peru towards the end of the 21 st century. Information obtained from these studies was incorporated, applying changes in meteorological variables (precipitation and temperature) and changes in glacier area in comparison to current conditions. Basin water contributions were thus obtained for two time periods: and Overall, results show a decrease of available water resources of up to 25% by the end of the century. Depending on the climate model and the scenario used, results show a reduction in flow during the wet months (in a few cases there are small increases). In relative terms, it can be seen that the decrease is greater in the dry season, which will accentuate the seasonal water shortage.

4 RCP2.6 RCP8.5 GISS-E2-H_p3-13.6% -19.9% MPI-ESM-MR -0.6% -8.9% GISS-E2-H_p3-17.7% -24.0% MPI-ESM-MR +1.6% -1.6% Table 1. Variation in annual average volume of contribution in comparison to the current situation The effect of glacier shrinkage is not very significant in the Cañete River Basin (less than 1% of annual averages). Thus, the greater impacts arise from future changes in meteorological variables (precipitation and temperature). However, in the case of the sub-basin draining into the Paucarcocha dam, the influence of glacier shrinkage on water availability is much more significant, particularly in the dry season. This case illustrates the phenomenon of reduction in future available water that may arise in Andean basins with a significant glacier surface. Finally, there is a certain dispersion in the results (figures 3 and 4) arising from the intrinsic uncertainties of the climate models used. Although the trend leads to an expectation of reduction in the basin flows, monthly changes may vary depending on assumptions. Variation in contribution Cañete basin (scenario RCP2.6) Change with regard to current monthly contribution (%) 20% 10% 0% -10% -20% -30% -40% JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC GISS-E2-H_p3 ( ) MPI-ESM-MR ( ) GISS-E2-H_p3 ( ) MPI-ESM-MR ( ) Figure 3. Percentage variation of future average monthly contributions in the Cañete River Basin for RCP2.6 in comparison to the current situation (orange shading indicates the range of possible results). Variation in contribution Cañete basin (scenario RCP8.5) 20% Change with regard to current monthly contribution (%) 10% 0% -10% -20% -30% -40% -50% GISS-E2-H_p3 ( ) MPI-ESM-MR ( ) GISS-E2-H_p3 ( ) MPI-ESM-MR ( ) Figure 4. Percentage variation of future average monthly contributions in the Cañete River Basin for RCP8.5 in comparison to the current situation (orange shading indicates the range -60% JAN FEB MAR APR MAY JUN JUL AUG SEPT OCT NOV DEC of possible results).

5 CONCLUSIONS The hydrological model of the Cañete River Basin was developed with RS MINERVE software, using data provided by CARE, Ambiand and CELEPSA as well as selected SENAMHI data. The model is divided, on one hand, into sub-basins and, on the other, into zones and subzones to facilitate its calibration and management of results. Calibration of the model was satisfactory for the purposes of the study. The calibrated model is a tool that enables the assessment of water resources at basin level and by zones and the analysis of future hydrometeorological scenarios which can be used to propose adaptation strategies for a sustainable use of water resources. There is a limited influence of the glacier parts in comparison to the non-glacier parts on the basin water contribution. This is due primarily to the relative extent of the parts: 15 km 2 of glacier surface and km 2 of non-glacier surface. Despite this, the glacier areas do play an important role in the water contribution that reaches the Paucarcocha dam, exceeding the nonglacier contributions during the dry season (when they reach up to 80% of the total contribution). The impacts of changes in meteorological conditions (precipitation and temperature in particular) and glacier shrinkage on future water resources have been analyzed both independently and together. Two time periods ( and ) and two scenarios have been considered. Once scenario is optimistic (RCP2.6) and the other, pessimistic (RCP8.5). The analysis indicates a greater water impact from future changes in climate. When meteorological and glacier conditions are incorporated into the hydrological model, there is a general decrease in water contribution in the Cañete River Basin for both time periods. The change will, however, be more marked towards the end of the 21 st century (from a 1.6% water increase with the optimistic scenario to a decrease of up to 24% with the pessimistic scenario). RECOMMENDATIONS The quality of the model enables a study of the most significant hydroclimatic processes in the Cañete River Basin. The model is, however, subject to potential improvements that depend on the availability of data recorded in the hydrological and meteorological stations. Improvement of the model is a continual process that must be adapted to the needs of each user. The studies of climate projections used suggest great uncertainty in forecasts. This uncertainty is reflected in the results of the impacts on water resources and should be taken into account when proposing adaptation measures that aim to be valid for all the proposed scenarios. Despite this fact, the global trend of decreasing flow is evident. The situation expected in the future for the Cañete River Basin is a decrease in the water availability, which vindicates the need for management tools adapted to each case under study. In order to know the impact of climate change on local needs for water resources it is necessary to carry out studies that assess local water demand. Once this information is available, it will be necessary to integrate it into the model and assess future changes in contributions to each point of demand. REFERENCES Bradley, R. S., Vuille, M., Diaz, H. F., & Vergara, W. (2006). Threats to Water Supplies in the Tropical Andes. Science, 312 (5781), Drenkhan, F., Carey, M., Huggel, C., Seidel, J., & Oré, M. T. (2015). The changing water cycle: climatic and socioeconomic drivers of water - related changes in the Andes of Peru. WIREs Water, 2(6),

6 García Hernández, J., Paredes Arquiola, J., Foehn, A., Claude, A., Roquier, B., & Boillat, J. L. (2016). RS MINERVE - Technical manual v1.7. RS MINERVE Group: Zurich. Gardner, A. S., Moholdt, G., Cogley, J. G., Wouters, B., Arendt, A. A., & Wahr, J. (2013). A Reconciled Estimate of Glacier Contributions to Sea Level Rise: 2003 to Science, 340(6134), Meteodat. (2016). Cañete: Escenarios de cambio climático. Informe. Proyecto Glaciares+: Lima. W. (2017). The freezing level in the tropical Andes, Peru: An indicator for present and future glacier extents. Journal of Geophysical Research Atmospheres, 122(10), Slater, A. G. & Clark, M.P. (2006). Snow Data Assimilation via an Ensemble Kalman Filter. Journal of Hydrometeorology, 7(3), Schauwecker, S. (2016). El futuro del clima y de los glaciares en el Perú. Informe. Proyecto Glaciares+: Lima. Taylor, K. E., Stouffer, R. J., & Meehl, G. A. (2012). An Overview of CMIP5 and the Experiment Design. Bulletin of the American Meteorological Society, 93(4), Schauwecker, S., Rohrer, M., Huggel, C., Endries, J., Montoya, N., Neukom, R., Perry, B., Salzmann, N., Schwarb, M., & Suarez, World Glacier Monitoring Service (WGMS). (2012). Fluctuations of Glaciers WGMS: Zurich. The development of this document was possible thanks to the Glaciares+ Project, an initiative of the Swiss-Peruvian cooperation within the framework of the Global Climate Change and Environment Program of the Swiss Agency for Development and Cooperation (SDC), executed by CARE Peru and the Swiss consortium led by the University of Zurich and including CREALP, METEODAT GmbH and EPFL. The project is carried out in close coordination with the National Water Authority (ANA), the Ministry of the Environment (MINAM) and the Center for Estimation, Prevention and Reduction of Disaster Risk (CENEPRED); and is implemented by the Water Resources and Glaciology Unit (UGRH) of ANA, regional governments of Ancash, Cusco and Lima, local governments and universities. Swiss Confederation Swiss Agency for Development and Cooperation SDC Partners PG+ Visualice y descargue el informe Modelización hidrológica Visualice y descargue el de la cuenca del Cañete y evaluación del impacto del Resumen Ejecutivo en cambio climático en los recursos hídricos. español.