1. Introduction
Groundwater proves to be an invaluable and priceless natural resource essential for various human activities, such as drinking water supply, industrial processes, and irrigation. Accounting for about 34% of the Earth’s freshwater, it forms the largest freshwater resource in the world and is immensely important for society [
23,
64]. Therefore, understanding the hydrological processes of this precious resource in a particular territory is necessary for supporting sustainable development.
The Sustainable Development Goal Six (SDG-6) of the United Nations aims to ensure universal access to clean drinking water, adequate sanitation, and improved hygiene, recognizing them as essential rights for all individuals by 2030 [
66]. Setting up this kind of target is a dire necessity in the ongoing unsustainable practices in the world, which are not only leading to quantitative reduction of surface water but also have made direct use of this water harmful for human beings, inviting various diseases. Further, the pace of rapid modernization, industrialization, and intensive agricultural practices induced by high population growth has also led to the shortage and increase in the demand for clean drinking water in both rural and urban areas [
18,
85]. Organic and heavy metal contamination has accumulated in surface waterways due to excessive and uncontrolled use, lowering the water’s quality. Meanwhile, global problems like drought and climate change have disastrously affected surface water, sometimes entirely destroying it.
These above-mentioned factors have led to humans’ dependency on groundwater to satisfy their daily needs. Subsequently, the agriculture sector accounts for the largest utilization of the groundwater resource in the world, i.e., around 42% of the world’s available groundwater, followed by household consumption for safe drinking water supply ranging between 25 % to 50% globally [
65,
71]. However, the proportion varies from country to country depending on the development level and the population’s needs. In India, over 90% of people in rural areas and around 30% in urban areas rely on groundwater for drinking and domestic purposes, causing over-exploitation in certain regions of the country [5, 6, 78]. In rural Bangladesh, tube wells are the primary source of drinking and domestic needs [
22]. In several rapidly developing sub-Saharan African nations, including Burkina Faso, the Central African Republic, South Sudan, Ethiopia, Chad, Nigeria, Somalia, and Uganda, groundwater serves as the primary water source for drinking around 70% to 90% of the local people [
12,
25]. In the United States, 38% of the population counts on groundwater for the purpose of drinking, sourced either from private wells or public systems, with self-supply wells catering to nearly 13% of the population [
34,
43]. In Nigeria, the dependency on groundwater is so high that only 20% of the Lagos population are provided with a piped utility water supply [
15]. With respect to agricultural consumption, approximately half of India’s irrigation activities rely on groundwater, making it one of the world’s largest water consumers. In China, around 9 million hectares of land are irrigated using groundwater [
30].
Around five million springs have been reported throughout India. Among those, around three million are in the Indian Himalayan regions and are experiencing depletion or seasonal fluctuation, attributed to diverse factors such as human interventions and the consequences of climate change [
54,
71]. The decline in spring water discharge can also be linked to rising temperatures, heightened rainfall intensity, altered temporal distribution, and a notable decrease in winter precipitation [
58].
The drying of springs is increasingly recognized as a critical issue in the Himalayan region, with a notable decline in spring discharge being documented across various areas [
71]. Research has highlighted severe water shortages due to the declining discharge of the springs, particularly during dry seasons [
33,
63]. A decline of over 30% in spring discharge over the past 30 years has been observed in Nepal’s mid-hill region [
8]. Additionally, about 50% of perennial springs in the Indian Himalayas have either turned out to be dry or become seasonal [
53]. Similarly, reports from the Himalayan state of Uttarakhand indicate a significant reduction in spring flow [
4,
29]. In addition, the transformation of about 82% of the spring fed streams of the upper Kosi river from perennial to ephemeral over the last twenty-five years is a critical issue [
57]. This drastic change has significantly reduced the lean flow of these rivers, creating serious challenges for the sustainability of the rivers and the communities that depend on them. Consequently, the local inhabitants are experiencing water shortages for drinking and irrigation, putting their livelihoods and the well-being of their livestock at risk.
The declining rate of ephemeral springs is notably faster than that of perennial springs in the Himalayan area [
51]. Consequently, understanding and managing the Himalayan region’s spring systems is challenging and crucial due to its accessibility. However, evaluating the capacity and productivity of springs in the Himalayan region is essential to understanding groundwater availability [
54]. The potential of springs indicates the probability of groundwater presence in regions with adequate water resources. In order to assess the potentiality of springs, the availability of groundwater has to be assessed, as it is the major source of water for the springs. For this purpose, remote Sensing (RS) and Geographic Information Systems (GIS) serve as efficient and cost-effective tools for generating valuable data on different thematic layers, aiding in the interpretation of potential groundwater zones [
36,
42,
45,
46,
47,
48]. RS techniques serve as a significant source of information regarding the correlation among slopes, landforms, land use, and lineaments. This data can act as an attribute in GIS and overlaid with additional datasets [
46,
72].
Recent studies have utilized various AI models to map groundwater prospect zones across different regions. In one study, Explainable Artificial Intelligence (XAI) was employed to assess fifteen groundwater conditioning factors using models like k-Nearest Neighbors (KNN), Neural Networks (NNET), Gradient Boosting Machines (GBM), Random Forest (RF), and Support Vector Machines (SVM). This study identified five zones with varying potential, ranging from very low suitability (14.95 to 300.24 km²) to very high suitability (969.49 to 1958.85 km²) [
80]. The study in Kanchipuram district, Tamil Nadu, India, used Landsat data and seven thematic layers to delineate groundwater potential zones, validating their map with high accuracy using the ROC method [
83]. A study conducted in Marathwada used frequency ratio (FR) and statistical index (SI) models across eight factors, with the SI model proving more effective for groundwater management and planning in the region [
84]. These studies highlight diverse methodologies and their application in enhancing groundwater resource management strategies globally.
However, investigators have utilized RS and GIS techniques to recognize potential groundwater zones by employing diverse thematic layers and field-observed data to validate their findings [
10,
16,
32,
38,
40,
47,
61]. However, the validations of groundwater potential zone maps with observed spring water discharge and perennial nature in a micro-watershed scale have not been explored sufficiently.
Therefore, the present study aims to delineate groundwater potential zones based on spring water discharge using RS and GIS techniques in the Khulgad micro watershed of the Kosi basin, Kumaun Lesser Himalaya. The demarcation of the potential zone mapping in a micro-watershed would be significantly useful for villagers, policymakers, and local government authorities to manage the potential zone of groundwater and conserve it at the micro-watershed level to meet the drinking water demand of the villagers.
2. Study Area
The Khulgad watershed is situated within the Kosi basin, located in the Lesser Himalayan terrain of India’s Almora district, within Uttarakhand State (Figure. 1). Covering an area of approximately 33 km², the watershed’s elevation ranges from 1080 m to 2140 m. The Syahidevi peak, locally known as the "water tower" of the Khulgad watershed, represents the highest point, contributing significantly to the area’s water resources. The drainage pattern of the watershed trails a dendritic pattern, with some abrupt diversions noted in major drainage channels due to faults traversing the watershed [
50]. Geomorphologically, the area comprises 24 types of fluvial, pluvial, and tectonic landforms [
50,
56].
The predominant land use/land cover in the area encompasses dense and semi-dense forests, primarily located in the southern and western parts of the watershed. Forested areas, mainly comprising oak and pine, cover approximately 52.2% of the total area, while barren land occupies 30.2%, agricultural land 13.7%, and urban areas 3.9%. The majority of the Khulgad micro watershed features shallow soil depths ranging from 25 to 55 cm, with sandy loam covering 50% of the area and the basin’s outlet identified as sandy clay loam.
The annual average rainfall in the basin ranges from approximately 980 mm in August to around 8.2 mm in November. Summer monsoon rains contribute approximately 75% of the total annual rainfall. The Khulgad micro watershed falls within a cool temperate climate region, with an annual average temperature of 20.3°C. The average maximum temperature was recorded as 26.7°C, and the minimum average temperature recorded was 15.3°C, respectively.
3. Hydrological and Geological Setting
The Khulgad watershed is situated within the northeast-dipping southern limb of the synclinal Almora Nappe, characterized by a thick folded sheet of Precambrian metamorphic rocks and associated granites [
50]. Extensively studied by various researchers [
20,
35,
50,
67], the geological map reveals six lithological units within the basin.
Augen gneisses formation covers approximately 18.8% of the area, predominantly comprising sheared and fractured gneiss formations in the southwestern part of the watershed [
50]. These gneisses pushed southward upon underlying schist and quartzite, have undergone extensive crushing, resulting in the development of secondary porosities that enhance permeability and form aquifers.
Garnetiferous mica schist, jointed quartzite, and friable quartzite, covering 18.8%, 28%, and 8% of the region respectively, from the underlying geological layers. Biotite schist and phyllite, constituting 15% and 8% of the watershed respectively, comprise the bottom layers (
Figure 1b). Hydrogeologically, the area is characterized by aquifers of secondary porosity, primarily consisting of faults, fractures, and joints. Structural geology plays a crucial role in understanding the characteristics of springs and groundwater in the region. The Sitalakhet thrust passing over the Salla, Naula, and Dhamas schist and quartzite formations is particularly significant, as it contributes to the formation of secondary porosity, making this zone a potential aquifer. The gneissic rock in the thrust region has become highly permeable due to the formation of fractures and joints, underlain by gougy clay acting as an impermeable layer. Consequently, the thrust areas of Sitalakhet, Salla, Champa, Naula, and Syahidevi hills form clusters of springs. The discharge of these springs is primarily dependent on rainfall and its variability, with climate change emerging as a crucial factor influencing the quantity and quality of Himalayan springs, sustaining their flow throughout the year.
6. Discussion
The identification of groundwater potential zones helps to understand the availability of groundwater resources in a specific area. The parameters for mapping depend on the physical conditions of the region. In this study, seven thematic layers, such as slope, lineament density, lithology, drainage density, rainfall, soil, and land use land cover, were prepared to identify the groundwater potential zones [
3,
7,
26,
54,
69,
70,
74]. Each thematic layer was assigned a weight from a 1-5 scale, the 1 indicates low value, and 5 indicates high value based on the hydrological conditions of the area [
54].
The rainfall is one of the primary inputs for the mountainous regions. Hence, the relationship between rainfall and groundwater potential zone mapping is a crucial aspect, especially the high rainfall was concentrated in the southwestern part of the study area, varying from 970 to 973 mm. However, it’s important to note that groundwater potential is not solely determined by rainfall, other factors, such as the water-holding capacity of the aquifer material, lineaments, geology, slope, soil, land use, and drainage density also play significant roles [
75,
79].
The lithology and secondary porosity directly impact the water-holding capacity of rocks in the study area [
56]. The field observations confirm that the gneissic rock in the southwestern part of the study area shows the highest amount of secondary porosity. Therefore, it has been assigned 5, the highest rank. The linear features in the watershed suggest that hard rock terrains transmit more water due to the presence of inter-connected secondary porosity [
17,
39,
79]. For this study, lineaments are digitized from satellite imagery (IRS-P6 LISS-3 FCC). However, both digitized lineaments and field-surveyed lineaments were used to understand the groundwater potentiality of the area.
In the Khulgad micro watershed, the steep slopes prevent the infiltration of rainfall into the aquifer system, particularly in areas with a very high degree of slope (>30 degrees), representing lower groundwater potential compared to other regions. Conversely, the southwestern part of the study area, characterized by lower slopes (0-10°, 10-17°, 17-23°), exhibits the highest groundwater potential. However, the northern part of the watershed (Kathpuria, Kurchon, Kaneli) shows higher steep slopes >30 degrees, indicating less groundwater potential, which was ranked as 1 and 2. Similarly, other researchers have explored the impact of slopes on groundwater potential in different terrains [
52,
55]. These studies revealed that steep slopes (>30 degrees) were linked to lower groundwater potential because of quick runoff and limited infiltration. On the other hand, slopes less than 15 degrees were associated with higher groundwater recharge [
55].
In the study area, soil has been ranked based on the texture and water-holding capacity, and the highest rank-5 has been assigned to the leptosol soil, and the moderate rank of 3 was assigned to the luvisol and chernozems [
86]. Similarly, for LULC, the highest rank of 5 was assigned to the forested land and low to very low for the buildup regions and roads in the study area [
87]. The drainage density of the watershed is also important to understand groundwater availability. The drainage density has been ranked 5 for very low drainage dense regions due to the less runoff and higher chance of infiltration, and the rank 1 has been assigned to high drainage dense regions due to the sudden runoff [
79,
82].
The current study utilized seven thematic layers integrated using the Analytical Hierarchy Process (AHP). However, the AHP has some limitations due to the subjective nature of assigning weights to the layers. To address this, we conducted an extensive literature review and field observations and sought expert opinions to determine the weights for each thematic layer. The consistency of the result can be checked by the consistency ratio (CR) value for the analysis. The CR value <10% (0.1) is accepted, and the weight and rank assigned to the parameters are justified and suitable for the AHP analysis [
75,
77]. In the present study, the CR value is 4% for seven parameters, which suggests the pairwise comparison is consistent.
The validation of the groundwater potential zones using RS and GIS with field observed data is always challenging in the absence of observed well yield and other available water resources information, particularly in the Himalayan regions [
54,
60]. In the current study, we followed similar approaches to substantiate groundwater potential zones. The occurrence, distribution, discharge of the spring, and density of the perennial springs are used to validate the groundwater potential area (
Figure 12). The validation revealed that about 50% of the perennial springs in the study area are located in excellent and very good groundwater potential zones. Geologically, this part of the basin is made up of fractured rocks of Gneiss and Quartzite, which is covered with thick soil and densely forested land. All these conditions are favorable for the high recharge in this part of the basin. Additionally, the ROC curve was employed further to justify the groundwater potential areas. Similarly, ROC curve methods were used by various researchers to validate the groundwater potential zone mapping [
19,
44].
This study has significant importance for local villagers regarding better groundwater utilization. This study provides a valuable resource for targeted water resource management and strategic planning. The policymakers can use this information to prioritize regions for groundwater extraction, implement sustainable water usage practices, and plan for infrastructure development, such as rainfall harvesting structures and spring water storage tanks to reduce the runoff. Additionally, the identified low potential zones in the eastern parts of the study area allow for better drought preparedness and management strategies, ensuring that water resources are allocated efficiently during periods of scarcity. Overall, this study helps policymakers and local villagers for implementing long-term strategies for sustainable water management.
Author Contributions
Nijesh: Conceptualization, Methodology, Validation, Investigation, Writing, original draft. Pant: Data curation, Methodology, Validation, Investigation, Writing, original draft. Anant: Data curation, Methodology, Validation, Investigation, Writing, review & editing. Abhinav: Conceptualization, Writing, review & editing. Abhinesh: Investigation, Writing, review & editing. Sury, Raju: Data curation, Methodology, Investigation, Writing, original draft. Rai: Conceptualization, Writing, Validation, Investigation, review & editing. Radha, Meera: Writing, review & editing.