Introduction

BACKGROUND AND RATIONALE

Importance of Water Quantification

The importance of water quantification has been inspired in this study by two overarching ideas: 1. Water supports all the services or benefits humans receive from ecosystems, and 2. Water scarcity is a growing concern for the subsistence of life on Earth.

Figure 1. Ecosystem Services Classification

Schema of ecosystem services grouped into four categories: Supporting, Provisioning, Regulating, and Cultural. This classification is enclosed within a circular blue line to illustrate the water's capacity to embrace the ecosystem service paradigm used to value and quantify the benefits humanity receives from nature.

Source: Adapted from [2, 3].

1. Water supports all ecosystems services

The ecosystem services are classified into four groups (Figure 1): provisioning, regulating, supporting, and cultural services [1, 2]. In this context, water is considered a cross-cutting resource that sustains all these ecosystem services. In other words, water can embrace the ecosystem service paradigm to quantify and evaluate the benefits humans receive from nature [3]. By quantifying water is possible not only to evaluate ecosystem services but also to find a trend in the availability of this natural resource that impacts human welfare and ecosystems’ functions.

2. Water Scarcity: A growing concern

It is estimated that only 0.4 % of global freshwater resources are available to support ecosystems and life on Earth (Figure 2). In addition, this percentage is not evenly distributed, causing water scarcity in some regions, which affects more than 40% of the global population [4, 5]. To face the water scarcity challenge, several techniques have been developed to estimate water availability and enhance water resources management. One of the most widely used is hydrological modelling, implemented in this study to quantify water at the sub-basin scale in tropical ecosystems.

Figure 2. Global Water Resources

A schema that illustrates the Earth's hydrosphere. About 97% of the total global water is stored in the oceans considered as not usable water. Only 2.5% is freshwater. However, from this 2.5%, 98.8% is considered not sustainable water because it is in the form of glaciers, ice caps, and groundwater which regenerates over long periods (geological time scale). From the remaining 1.2% of freshwater, 0.8% is permafrost that only can be used if it melts and only 0.4% can be considered renewable water (regenerates in short periods) available to support ecosystems and life on Earth.

Source: [4].

Figure 3. Global Hydrologic Cycle

Illustration of the hydrologic cycle where the primary input is the precipitation and the two outputs, the evapotranspiration, that is, water returning to the atmosphere and the water yield, which is the liquid water output accessible for management and the main variable used in this project (bottom map). To complete the cycle, the intermediate processes illustrate the water transportation and phase changes.

Source: Adapted from [6].

How to quantify water availability?

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HYDROLOGICAL MODELLING

Hydrological modelling is an instrument that simplifies the hydrologic cycle (Figure 3) and allows the quantification and evaluation of different biophysical and socio-economic factors that influence hydrological processes in a watershed. Additionally, hydrologic models allow evaluating the current state of water resources and trends based on historical changes and future projections.

What to estimate? → WATER YIELD

Water Yield is the output obtained from the hydrologic cycle (Figure 3). Therefore, water yield is a proxy of the water availability in a region. It is estimated through hydrological modelling by introducing climate (precipitation and temperature) and geospatial features (topography, soil, and land cover) of a watershed system.

Climate information (precipitation and temperature) is the main input for water yield estimations. However, hydrologic models require daily measurements from meteorological stations, which is rarely available, especially in protected areas in tropical ecosystems where accessibility is limited. On top of that, estimations are generalized to vast zones without measurements. Thus, in this project, I will consider water availability modelled from limited climate data and analyze its statistical correlation with historical climate data from interpolated spatial models that can better represent the reality of each individual sub-basins in the whole drainage area of study.

RESEARCH QUESTIONS AND APPLICATIONS

With the water yield estimations at different sub-basins, the main objective of this study is to statistically assess how biophysical factors, such as climate and land cover, can influence water availability. To do so, the following research questions are posed:

Research Question 1

How are climate change factors such as mean annual precipitation and temperature related to the water availability (water yield) trend?

Application:

If water availability trends are explained by climate change only, the regression models will offer quantitative support for water management policies. However, this analysis could also open to other research questions related to the contribution of other factors that might influence water availability, as considered in the next research question.

Research Question 2

Has land cover change between decades positively or negatively affected the water yield behaviour in the whole drainage area?

Application:

Several studies reveal that deforestation increases water yield while reforestation decreases it [8]. Therefore, analyzing the statistical correlation of the land cover change with water yield will shed light on forest recovery policies' impact on water availability.

RESEARCH IMPACT

Water availability is estimated at the drainage region generated at the Conservation Area of Guanacaste (ACG for its acronym in Spanish) in the northwestern part of Costa Rica. The United Nations Educational, Scientific and Cultural Organization (UNESCO) declared the ACG as a World Heritage Site in 1999 for the ‘Outstanding Universal Value’ of its tropical ecosystems' complex ecological dynamics [7]. Some examples of the ACG's astonishing ecosystems are shown in Figure 4.

Therefore, understanding factors that influence water availability in such an important place, would offer a scientific base for rational decision-making regarding conservation strategies of forests and monitoring of climate change to develop adaptation strategies to guarantee water provision for the sustainability of the ecosystem services and local populations.

Figure 4. ACG Landscape

Examples of different ecosystems found at the Conservation Area of Guanacaste (ACG) - Costa Rica: a. Cloud Forest, b. Savannas, and c. Dry Forest.