Environmental flow determination by hidrological modeling

Introduction

The world water legislation has a common challenge: distribute fresh water resources in a fair, efficient and sustainable way. Traditionally, agriculture, industry, mining and municipalities have competed for these limited resources.

The old legal structures were created to satisfy economic and domestic use of water demands ignoring the ecosystem use that, until recently, were not fully scientifically understood. As a result of this approach, not included to the environment, a lot of rivers and creeks are not the same today as the water and ecosystems they used to be. Water use in agriculture, industry, mining and municipalities have manipulated and degraded fresh water resources in an abrupt way on the environment.

Water cycle in Andean basins

Water resources cycle for Andean basins have two marked seasons: dry and rainy, aside of transition periods named wet seasons, as shown in figure 1.

Figur 1.Vegetation cycle for an Andean basin. 

The agricultural activities are done in the rainy season and wet season, while in dry season natural vegetation decreases to concentrate only in wetlands and groundwater discharge zones.

In Andean basins, during rainy season, there is no restriction for the ecosystem due to the big amount of water coming from precipitation and river flows. Wet season still has important events of precipitation, therefore, rivers possess intermittent discharges. In dry season, all the discharge comes from groundwater storages because precipitation events are limited and soil doesn´t saturate. It is during wet and dry seasons where the analysis and determination of environmental flow is necessary for quality water, the biotic interactions and river habitats maintenance.

Legal framework

The law  N° 29338 not only regulates water use as a resource but also the assets associated with it, as these natural (marginal strip, channels, carrying material, glaciers, etc) or artificial (uptake, storages, conduction, measurement, sanitation, etc).

The law saves the principles of valuation of water and the integrated water management contemplating its sociocultural, economic and environmental value. It is considered that water is part of the ecosystem. This law prevents affectation of the natural conditions of water resources.

Water resources law regulation is N° 001-2010-AG decree. The article N° 153 is about environmental flow. The regulation defines the environmental flow as “the water flow to be maintained in the natural water sources for the ecosystem involving protection or conservation, the landscape aesthetics or other scientific or cultural interest”.

Proposed work plan

With the climatic records, satellite images, field compilation and records of water availability gauging inside the basin corresponding to the study zone, the hydrological response is represented in a PRMS distributed numerical model.

The distributed numerical modeling allows a quantitative understanding of water cycle in the basin correspondent to the study zone. The temporal extension of the model is determined on a period of years that represent the average water cycle.

Hydrological modeling is calibrated to correspond some physical process values of water cycle modelated with observed data field. Calibration parameters is the precipitation distribution, evotranspiration distribution and runoff distribution.

The process of calibration is focused in uncontrolled basins hydrological answer, and then the answer of all the basin is interpreted in natural conditions before manipulated for dams and bypass channels. With the flow results of the numerical model, the average recession curve corresponding to average flow in the basin during dry and wet season is analyzed.

Numerical tools

The PRMS modeling code (Precipitation-Runoff modeling system) is a spatially distributed parameters modular system that represents physical processes of a hydrographic basin. It was developed by the United State Geology Survey – USGS to evaluate the effects of several geomorphologic combinations, type and use of soil, vegetation and climatic parameters over the hydrological response on a basin. The response to precipitation as rain and snow, in a regular form, and to the extreme precipitation events can be simulated to evaluate changes in water balance, flow regime, the peak flow and volume, the relation between soil and water, and the recharge of groundwater.