<p>In Colombia, around 70% of the energy is from hydropower source and most of its associated hydraulic infrastructure is located in the macro-basin of the Magdalena and Cauca rivers. These projects are generating a drainage network fragmentation and a substantial alteration in natural regimes of both flow and sediment, which is having negative impacts on freshwater ecosystems and their ecosystem services. Nowadays, there is a methodology related to environmental flows, proposed in 2018 by the Colombian Ministry of Environmental and Sustainable Development (not adopted as a regulation), which integrates two sets of stakeholders: environmental authorities for integrated water resources management at regional scale and holders of new projects with high impact on the natural flow regime of rivers. Since it is only a non-binding methodological proposal, there are no known practical applications to evaluate its effectiveness in terms of reduction of environmental impacts, social conflicts, and water governance. In this sense, we propose a step forward in the analysis of the effects of its application in rivers with available hydrological data and under natural regime conditions through a taylor-made computer model (HeCCA 1.0) which is composed of the most important methods contemplated in the above-mentioned methodology.</p><p>HeCCA 1.0 allows determining the percentage of monthly use of a river without compromising its ecosystem function, based on it, river discharge data of 15 different watersheds located throughout the entire country were used. In this test, a range of drainage areas (180 to 73000 km<sup>2</sup>) was covered located between 25 and 2993 meters above sea level. The systems belong to different seasonal behaviors depending on the geographical location (monomodal or bimodal). For the monomodal regime, utilization percentages were obtained between the 61%8 and 77%29; and for bimodal regimen between 14% and 49%32.</p><p>These results depend on the geographical location of the basin, the watershed size, if it is related to the runoff seasonality along the year in the different catchment areas of the country. The taylor-made computer model provides stakeholders a holistic overview of the water availability and management, giving quantitative tools for an optimal development of water governance in the region.</p>

<p>In Colombia around 70% of the electricity generation is from hydropower. It is documented that their infrastructure and operation rules affect the natural regime of flows and sediments with several impacts on aquatic ecosystem functioning mainly on the Magdalena-Cauca river basin, where most of the projects are located. Also, social conflicts have been documented downstream hydropower projects due to water use incompatibilities. Considering that Colombia has a great potential to expand hydropower generation as well as there is a growing demand from other water users, it is necessary to attend the ecological requirements of aquatic ecosystems and to improve the water management in order to avoid irreversible environmental impacts and governance problems.</p><p>In 2018, the Colombian Ministry of Environmental and Sustainable Development (MADS) developed a methodology to consider environmental flows both in the water management decisions and in the environmental impact assessment of new projects with impacts on hydrologic regimen. The opportunity to carry out a validation of its premises aims to research its effectiveness in terms of reduction in hydrologic alterations when environmental flow allocation is decided. </p><p>That is why we have developed a computer model (HeCCA 1.0) which contains the most important methods contemplated in the methodology proposed by MADS. Thus, using river discharge data of 15 different river systems located throughout the entire country, the methodology mentioned has been tested in basins with low anthropic alteration of the hydrological regime. In this test, we cover a range of drainage areas, from 180 to 73000 km<sup>2</sup>, located between 25 and 2993 meters above sea level, and different climatic and geomorphological characteristics. </p><p>The following results have been obtained using the HeCCA tool. For the 15 river systems, the statistical quartiles Q1, Q2 and Q3 for the percentage of use are monthly correspond to 24%, 47% and 100% respectively. The systems belong to different seasonal behaviors depending on the geographical location; nine of them count on a monomodal regimen, which average percentage of use is 61%±8, and the highest percentages of use (located in the Pacific basin) are not found during the wettest months, (77%±29); four of the watersheds are in the Orinoco basin, providing use of water between 61% and 67%. Six systems have bimodal regimen, whose average percentage of use is 49%±32, the two lowest percentages of use (14% and 19%) are found in the biggest bimodal watersheds, with sizes over 1700km<sup>2</sup>, which also have the highest average yields. The highest percentage of use found during the wettest months of the year is 99%, corresponding to the system located at one of the lowest points of the Caribbean basin.  </p><p>Thus, the percentage of available water depends on the watershed size, if it is related to the runoff seasonality along the year in the different catchment areas of the country. This approach provides stakeholders a clear overview of the water availability and management through a useful tool which improves the integral water management for hydrological systems.</p>