THE HYDROLOGICAL RESPONSE OF SURFACE WATER ON KIRITIMATI ISLAND, KIRIBATI TO RECENT CLIMATE VARIABILITY

For small tropical islands with limited freshwater resources, understanding how surface hydrology is influenced by regional climate is important considering projected extremes in hydrologic variability, amidst growing populations. However, piecing together local-scale climate variability and hydrologic variability for islands in the tropical Pacific remains a challenge due local hydroclimatic data scarcity and island remoteness. In the absence of continuous on-the-ground hydrological measurements, we derive time series of surface water area on Kiritimati, Republic of Kiribati (2°N, 157°W) from Landsat data with the normalized difference vegetation index (NDVI) using GRASS 7.0. The time series show that the surface water area varies seasonally, following the island wet and dry seasons, and also interannually, due to hydroclimate variability associated with the El Niño/Southern Oscillation. The Niño 3.4 sea surface temperature (SST) index, satellite-derived precipitation, and local sea level all had significant positive correlations (R=0.56, 0.38,0.41, p=0.05, N=81) with the surface water area dataset, whereas correlations with evaporation were not significant, and correlations with precipitation–evaporation (­P–E) were significant, but weak (R=0.12). Accounting for the influence of climate in preceding months, correlations between surface water area and SST were significant for 5 months prior, sea level for 4 months prior, and 3 months prior for precipitation, suggesting that the duration of the wet season and the frequency of El Niño events may be more important than amplitude for maintaining water storage on the island. Variable correlations between island sub regions and climate indices suggests that surface hydrology on Kiritimati is not uniform in response to climate, but rather aspects of island geology determine the extent to which climate controls variability of surface water area. The results of this study suggest that future changes to the frequency of interannual hydroclimate variability or shifts in seasonal duration and strength may have significant hydrological consequences, altering the timing and magnitude of surface water coverage on Kiritimati, which has implications for freshwater resources, flooding, and drought for the growing population of Kiritimati.