## 2 Materials and Methods  
### Study Area  
The Neretva River Basin is located in Bosnia-Herzegovina (B&H), with its delta and mouth in Croatia (Figure 1). The Neretva River is of great socioeconomic and environmental importance to B&H and Croatia. More than 350,000 inhabitants live in the watershed. The river water mainly flows through karst areas and exhibits typical karst hydrological characteristics [4; 12]. The length of the river is approximately 240 km, and the basin size is approximately 10,100 km\({}^{2}\). The river has more than 30 permanent and occasional tributaries. The average height of the basin is 250 m above sea level (ASL), and springs are located at a height of 1095 m ASL [15]. The climate is Mediterranean in the lowland area close to the sea,whereas the middle part has a continental climate, and the higher area has a mountainous climate. Temperatures range between \(-29^{\circ}\) C and \(43^{\circ}\) C, and the annual evapotranspiration is 500-900 mm. The average rainfall is 1650 mm. Most of the precipitation occurs in winter (November), while summer (July) is often without precipitation. The average discharge is 269 m\({}^{3}\)/s, with a minimum of 44 m\({}^{3}\)/s (probability: 0.05) and a maximum of 2179 m\({}^{3}\)/s (probability: 0.1) [15]. The mean annual runoff coefficient is high, at approximately 0.65.  
The river has three characteristic functional zones--namely, a steep upstream zone that forms the productive zone, a milder lowland zone with a small drop at the bottom of the bed that forms the transit zone, and a delta zone with an extremely small drop that forms the accumulation zone. Nine hydroelectric power plants have been built in the upper stream, and several more are planned [4]. In the middle course, from Capljina to Metkovic, the river flows through alluvial valleys (4-6 m ASL). Owing to its fertile soil, abundant water, and favorable climate, the valley is used for agricultural production. The river valley narrows significantly near the village of Gabela, where a bottleneck effect occurs, and then widens again into a valley and delta situated in Croatia. This is called the Neretva Delta or just the delta (Figure 1). In 1887, the river was regulated from Metkovic to the sea for navigability and flood protection, and the delta was reclaimed for agricultural and urban areas [16].  
The delta covers an area of 156 km\({}^{2}\), and the river is 19 km long. The entire area is relatively low, from 1 to 6 m ASL. In the past, most of the area was flooded, forming a land delta system built from fluvial sediments in a standing body of water. The delta is a major source of WEF resources. Many coastal and underwater karst springs at the edge of the karst valley recharge the surface water in the delta [4]. The area is ecologically significant and, together with the upland area in B&H, forms important coastal marsh areas in the Mediterranean. Since 1992, the peripheral parts of the delta along the karst masss have been an ethylological and ornithological reserve under the protection of UNESCO; in B&H, it is Hutovo Blato (since 2001). In the ornithological reserve, the local species \"Blue Eye and Lake Desne,\" \"Usce Neretva,\" and \"Kuti\" are protected.  
The Neretva Delta has the highest production of citrus fruits in Croatia, as well as winter and early spring crops. The plain has more than 70,000 inhabitants living in several settlements, the largest being Metkovic and Ploe, the second-largest sea harbor in Croatia. Water management is technically and politically complex because numerous settlements and socioeconomic assets have developed in the delta area, which are increasingly threatened by climate change and the rise in the MSL.