Use of Soil Moisture as an Indicator for Climate Change, Using the SUPer System

1. Introduction

A natural disaster is any calamitous occurrence generated by the effects of nature, in general, and then human-driven phenomena that produce great loss of human life or destruction of the natural environment, private property, or public infrastructure. A natural disaster may be caused by weather, climate events, landslides, and other occurrences originating at Earth’s surface [15]. Some types of disasters are often limited to or occur more frequently in specific geographic regions. For example, in Pernambuco, two specific geographic regions have landslides, flooding, or drought (semiarid). The rains generate river floods and flash floods, compromising the quality of life of the local population and causing significant damage to the economy in Pernambuco state [20].

From 2001 to 2018, more than 5000 water-related disasters occurred globally, accounting for 73.9% of all natural disasters. These disasters (i.e., floods, storms, landslides, and droughts) caused over 300,000 deaths and ~1.7 trillion dollars in economic damage worldwide. Floods and droughts have especially significant financial repercussions in recent history. In the 20th century, an estimated $365 billion was lost worldwide due to approximately 2100 floods and 430 droughts. The 21st century has experienced an increased frequency of water-related disasters and their associated consequences: from 2001 to 2018, around 600 billion dollars were lost globally due to more than 2900 floods or 290 drought events [21].

Unfortunately, the frequency and intensity of natural hazards and extreme events has increased over the last century, with reliable data showing a clear increase over the last four decades. The increase has been attributed to numerous factors, including climate change, world population growth, human activities, and ecosystem degradation [4,14]. Climate change has led to increased the frequency and intensity of hazardous events, partly related to the trend towards an increase in temperature and the frequency and intensity of precipitation events [21]. Human settlements are increasingly occupying areas of the landscape that are naturally prone to hazards, including floodplains, low-lying coastal areas, and alluvial fans [21]. In addition, changes associated with the landscape's physical, chemical, and biological characteristics, largely due to human activities (including urbanization and deforestation), have resulted in widespread degradation of ecosystems. This degradation leads to a decrease in the ability of nature and society to recover from a natural hazard [18], thus creating a positive feedback loop that drives an increase in the more significant number of devastating disaster events. Associated ecosystem functions essential for human well-being, including the ability to grow food and supply fresh water, are also degraded [21].

Throughout history, Pernambuco has recorded different problems related to rains and droughts. On the coast, there are many problems with flooding, and in the semi-arid, there are many problems related to severe droughts. In 1975, Recife, the capital of the Pernambuco state, had 80% of the inhabited territory flooded. There were five days of damage - with hospitals operating by candlelight and transportation through the neighborhoods being improvised in boats. In 2022, after almost 50 years, a new major disaster occurred in Recife, Landslides. Recife, the capital of Pernambuco, is a low-elevation city susceptible to flooding. Pernambuco was the worst tragedy in the last 50 years due to natural disasters. The event resulted in 130 deaths, more than 119,000 homeless, and 44 cities with an emergency decree. In Pernambuco, there is frequent flooding in the coastal region. On the other hand, in a semiarid region, in 2021, twelve cities of Pernambuco were in an emergency because of natural disasters and drought. Soil moisture is important for monitoring natural disasters related to droughts and floods.

Fire has been used for many years in Brazil, serving various purposes. However, if mishandled, it can cause fires with immense environmental damage. The Sertão and São Francisco Pernambucano mesoregions are susceptible to fires since the predominant biome is the Caatinga, with a semi-arid climate and low rainfall [8].

Hydrology, as a science, refers to studying the hydrological cycle. This comprises hydrological processes that can be analyzed over time, space, or both together, which, in turn, are quantitatively described by hydrological variables and represented by historical series. Therefore, it is essential to screen historical series to properly assess hydrological processes and their variability to public action policy [9].

The hydrological model Soil Water Assessment Tools- SWAT is very widespread in Brazil and due to the need for adjustments in the results obtained through modeling and observed data, it is necessary to carry out a calibration. Different studies have been developed to calibrate the swat model to Brazil, [2,13,23,27] northeast of Brazil, [11], and for the Pernambuco basins [1,5,11,24–26]. The intention was and is to provide a system that can facilitate the monitoring and planning of watersheds. This system is called Super- System of Hydrological Response Units for Pernambuco [6,7,12,26,28,29].

Soil moisture plays a crucial role in mitigating the impacts of natural disasters, particularly in water-related hazards such as floods and droughts. Understanding and effectively managing soil moisture levels can contribute significantly to disaster risk reduction in the following ways: flood management, drought resilience, landslide prevention, water supply management, and ecosystem health. Therefore, studying and monitoring soil moisture is crucial for better natural resource management, water resources, and mitigating the impacts of climate change. This study aims to relate dry and rainy years with soil moisture in the basins of Pernambuco and to show the impacts of natural disasters that cause droughts and floods in Pernambuco.

2. Materials and Methods

In this study, we selected two basins for analysis. The first, Capibaribe, is a wetland basin that covers part of the capital, Recife. The Capibaribe basin has an area of 7,536.65 km2. The second, Terra Nova, has 4,909.09 km², is a semi-arid basin that receives water from the São Francisco River through the PISF-São Francisco Integration Project. Hence, these two basins were chosen to be present here.

Figure 1. Localization of the basins Capibaribe (black) and Terra Nova (red), Pernambuco, Brazil.

Figure 1. Localization of the basins Capibaribe (black) and Terra Nova (red), Pernambuco, Brazil.

This study used the SUPer system (System of Hydrological Response Units for Pernambuco), https://super.hawqs.tamu.edu/. SUPer is a simulation model for the quantification and quality of water in the basin. More detail for SUPER can see in [5,12,26,29].

The methodological basis of SUPer is the Soil and Water Assessment Tool (SWAT). SUPer has been used to simulate numerous basins in the state of Pernambuco for soil and water resource assessments. SUPer is an efficient model that simulates various watershed processes for a defined recording period. SUPer allows you to estimate soil moisture in spatial and temporal detail. Thus, the SUPer is believed to be very useful in indicating disasters that have a good relationship with the water balance and soil moisture.

In the super there are several variables of the hydrological and environmental cycle. This study shows the calibration of the SWAT model for the Capibaribe and Terra Nova basins that were included in the SUPer. In the SUPer the model was run in daily scale, the set-up/warm-up was 2 years and SWAT model version was SWAT2012. The project in SUPer to Capibaribe basin has subbasins 53, HRU-Hydrology Response Unit has 253 and Terra Nova basin has subbasins 32 and HRU has 174.

The data used in this study are SW- Soil Water, PRECIP-Precipitation of the period 1961-2021, where the years 1975, 2000 and 2005 were highlighted.

Scenery

• Actual
• Semiárid decrease 15% rainfall and increase 2celius in temperature (climate change)
• Umid region Increase 17% rainfall and increase 2celsius in temperature.

This study used aerial photography images obtained with a UAV to visualize changes in vegetation and surface temperature in areas with changing conditions in the São Francisco River water diversion project. That is an area with water entering the hydrological system.

3. Results

Capibaribe basin

It is possible to observe important changes in the average annual water balance, Figure 2 and Figure 3. But, is very important to see return flow because increase 27%. It’s possible see increase percolation, increase surface runoff, Increase return flow. Consequences, for example, reservoirs often with high volume. Attention! dam safety.

Soil moisture Capibaribe-low-SUPer basin

Climate change is already happening, in local scale. Climate change increase soil water, actual even more in the future, Figure 4. Soil moisture will be further impacted in the future. Consequences: Increased likelihood of landslide. The humidity data it is possible to make a prediction of landslide alert due to the high humidity content. Like for example in 1975, Figure 5. The capital of Pernambuco has faced two major floods in the last half century, the first in 1966 and the second, and more significant, in July 1975. Between July 17 and 18, 1975, streams and the Capibaribe River rose rapidly and overflowed, flooding entire neighborhoods. About 80% of the city of Recife was underwater. Very attention in this period. is more security for saving lives than only weather forecasts.

Between July 30 and August 1, heavy rains punished the state, including the Metropolitan Region of Recife, leaving a total of 22 dead, 100 injured, and more than 60,000 people homeless. Cities were partially destroyed, and the waters that overflowed from the rivers washed away bridges and houses. Almost all year round the soil was saturated, Figure 6.

Between May 30 and June 2, heavy rains caused flooding in 25 ages of the Agreste, Zona da Mata and Littoral of Pernambuco, leaving 36 dead and more than 30,000 people homeless. About seven thousand houses were partially or destroyed; 40 bridges were damaged; 11 state highways were hit, seven of which were closed. The water flooded several cities' central streets, hospitals, schools, and commercial houses, causing enormous material damage. In this period, the SW is shown in Figure 7. Yet, soil moisture (SW) and precipitation can separate flooding risks and landslides.