Spatial and temporal variation of papyrus root mat thickness and water storage in a tropical wetland system.

Papyrus wetlands are predominant in permanently inundated areas of tropical Sub Saharan Africa (SSA) and offer both provisioning and regulatory services. Although a wealth of literature exists on wetland functions, the seasonal behaviour of the papyrus mat and function in water storage has received less attention. The objective of this study was to assess the response of the papyrus root mat to changing water levels in a tropical wetland system in Eastern Uganda. We delineated seven transects through a section of a wetland system and mapped wetland bathymetry along these transects. We used three transects to measure spatial and temporal changes in mat thickness and free water column, and to monitor variations in total depth during two seasons. The free water column increased across all transects in the wet season. However, changes in the mat thickness varied spatially and were influenced by the rate of increase of the free water column as well as wetland bathymetry. The proportion of mat compression was higher at the shallow end of the wetland (83%) compared to the deep end (67%). There was a significant negative correlation between changes in free water column and papyrus mat thickness (r = -0.85, p = 000). Therefore, the mat compresses in response to increase in free water column, which increases the ratio of the free water column to root mat thickness. Hence, the wetland accommodates excess water during rainy seasons. Water depth varied from 1.5 m to 2.1 m during the monitoring period, corresponding to a water storage of 61,597 m3 and 123,355 m3 respectively. This means a 50% change in water volume for the studied wetland section. This water regulatory function mitigates severity of floods downstream, but the stored water is also useful to the surrounding communities for wetland-edge farm irrigation during dry seasons.