An Analysis of the Effects of Large Wildfires on the Hydrology of Three Small Catchments in Central Chile Using Tritium-Based Measurements and Hydrological Metrics

Francisco Balocchi,Donald A White,Diego Rivera,José Luis Arumi,Uwe Morgenstern,Pablo Ramírez De Arellano,Richard P Silberstein

doi:10.3390/hydrology9030045

Abstract

Wildfires are an important disturbance affecting catchments’ soil and hydrological processes within. Wildfires are predicted to increase in both frequency and severity under climate change. Here, we present measurements of tritium (3H) in surface water of three streams before and after the ‘las Máquinas’ megafire of January 2017 in central Chile and streamflow metrics. Mean transit times (MTTs) of water were calculated in three coastal catchments with the Mediterranean climate type, covered by native forest, a mixture of native forest and Pinus radiata D. Don, and P. radiata. Lumped parameter models (LPMs) were used to obtain MTTs. Tritium activities from 2012 to 2018 ranged from 0.597 to 0.927 Tritium Units (TU), with the lowest TU activity in 2018. These 3H concentrations indicated water ages from 5 to 30 years. Following the fire, peak flows and baseflow have increased in two catchments but decreased in the third. Even though we have seen changes in the hydrological responses within the three catchments, pre- and post-fire MTT values were not significantly different. Therefore, there is no conclusive evidence of hydrological changes at the groundwater level due to wildfire at this early stage. However, since the MTT ranges from 5 to 30 years, it is likely that more time is required for the changes in the hydrograph to be clearly reflected in the tritium signal even though there are noticeable changes in streamflow metrics such as runoff and baseflow. Within the following years from this study, a sampling schedule to continue to investigate both the long-term drought and the effect of wildfire on these catchments will be maintained.

Highlights

Management of the effect of fires on the supply of water to regional communities requires knowledge of their effect on forest structure, soils, and hydrogeology and on the local and regional water balance
This study reports tritium isotopic composition of water in the stream and rain in these burnt catchments before and two years after the fire and uses these data to test the hypothesis that the Mean transit times (MTTs) of water in these catchments is not affected by wildfire
As noted by using hydrological metrics, there was no noticeable pattern of change in MTT due to the fires in this catchment, and there was no significant difference in MTT between the water source and outlet

Summary

Introduction

Management of the effect of fires on the supply of water to regional communities requires knowledge of their effect on forest structure, soils, and hydrogeology and on the local and regional water balance. The most intense and damaging fires occurred in the Maule Valley, east of the coastal city of Constitución, and have become known as the Las Máquinas fires. While they were historically severe, it is expected that climate change will further increase the frequency and intensity of wildfires in Central Chile [2,3]. These fires occurred principally in the coastal range of Central Chile. The water that flows to the Maule River is a regionally important supply of drinking water and irrigation and is important to the many rural communities in the ranges

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