The climate of the Gambia is generally described as Sudano-Sahelian. Rainfall is concentrated in the wet season between the months of May to October with an average of 850mm per annum. The rainfall is generally less in the northern half of the country and greatest in the south-west. There has been a reported 25 to 30% decrease in annual average rainfall over the period 1950 – 1990 (DPWM, 1997). No rain gauges are located within Niumi National Park, the closest being based at Fajara on the south bank of the River Gambia, under the supervision of the Gambia-German Forestry Project.
The bolongs or creeks of Niumi National Park are subject to the daily rhythm of the tides, which have a maximum range of 2m in equinoxal springs. During the rains from May/June to October/November, the salinity of the bolons decreases in the upper reaches. Low-lying areas on Jinack Island and on the mainland flood during the rains creating generally linear ponds and seasonal marshes in the salt pans and Tamarisk, Tamarix senegalensis scrub. The degree of flooding is a function of the extent of the rainfall, and in low-rainfall years flooding may be limited to relict bolons. Flooded areas gradually recede as the dry season sets in and salinity of the water increases through evaporation (Ramsar Wetland Study The Gambia, 1997). The ground water table on the island of Jinack fluctuates between 3 and 5m depending on the season. The complexity of the aquifer in this area has not been thoroughly surveyed, but in places freshwater is present at 3 to 22 4m depth in less than 100m away from the shoreline (Ramsar Wetland Study The Gambia, 1997).
Niumi National Park occupies the southern portion of the Sine-Saloum Delta and has a surface geology greatly influenced by the formation of a Ria (drowned river valley) within the Niji Bolon during the Noukachottian transgression. During this transgression sea level rose 3 to 4m, flooding much of what is now Niumi National Park, resulting in sequences of unconsolidated sand, silts and clay. Beaches and sand dunes were subsequently left perched 4m above the existing high water mark, though they have subsequently been eroded and reworked. The sand deposits on the oceanic coast are referred to as Coastal Beach Complex. The Continental Terminal Series abuts onto this complex at Niumi with occasional exposure to laterite boulders as in the escarpment east of the Masarinko Bolon. The Nouakchottian shoreline is evident along the Ker Saniang Bolon where it forms low eroding cliffs. Jinack island and the mosaic of islands to the north which form the Sine-Saloum Delta, are essentially shifting shoals of sand which have stabilized through colonization by vegetation though still maintain a degree of dynamism evident in the erosiondeposition occurring at the channel mouths eg Buniada Point (Ramsar Wetland Study The Gambia, 1997).
The average elevation within the park is less than 5m, with a maximum of approximately 15m. The high ground occurs primarily along the Masarinko Bolon where a sandy escarpment fringes what presumably was an ancient shoreline. e.
A detailed soil survey (Ramsar study, 1996) as conducted in the vicinity of Kajata and Niji on Jinack islands as part of a program to rehabilitate rice fields that have been subject to saltwater intrusion over the last decade. The survey area was confined to two broad physiographic units: the elevated sand dune complexes and the low-lying flood plain area. The sand dune complexes are composed of relatively young underdeveloped, coarse textured soils that are low in nutrients and available water. The soil is unsuitable for most crops with the exception of coconuts (cocos nucifera). The low-lying flood plain is loaded with sodium salts, though the soils are moderately drained and contain appreciable amount of nutrients. The problem of excessive sodium could be overcome if sufficient water is available to leach it out (Ramsar Wetland Study, The Gambia, 1997)