Abstract:
Hydrological performance of Upper Mahaweli Undulating Catchment (UMUC) has been
studied to a lesser extent in Sri Lanka. This study focused on long term (2005–2024)
hydrological and climatic data of three sub-catchments: Peradeniya, Nawalapitiya and
Thaldena. Here, Peradeniya catchment showed the greatest variability in water levels (1–4
m) and discharge rates (9–244 m³ s⁻¹). Maximum flow values greater than 240 m³ s⁻¹ are
observed. Nawalapitiya catchment area shows moderate water levels and discharge patterns
(0.4–2 m, 1–62 m³ s⁻¹). It faces the challenges of a semi-urban, agricultural and mountainous
geographical location. Thaldena catchment area showed the most stable water levels and
discharges (0.1–1.2 m, 1–61 m³ s⁻¹). Although, UMUC cover different climatic zones, it
shows the early signs of risk such as subtle increase in maximum flows in recent years.
Pearson correlation analysis showed a positive relationship between water levels and
discharge in Peradeniya (r = 0.992) and Nawalapitiya (r = 0.903). However, for the Thaldena
catchment, the correlations with other catchments were weak or negative and different from
the other two sub-catchments, may be due to different climatic zone. The runoff-rainfall ratio
values were at a median level in Peradeniya (67%) and the lowest in Thaldena (19%) during
the period of 2010–2020, suggesting the efficiency of reverse flow and infiltration capacity.
Nawalapitiya catchment showed a most high runoff-rainfall ratio (82%) during 2010–2016
period. Comparing three sub catchments, Peradeniya showed high variability, strong water
level–discharge links, rapid runoff, and high flood risk from urbanization. Nawalapitiya has
moderate, stable flows and moderate flood risk. Thaldena remains stable with low runoff,
weak correlations, low flood risk, and high groundwater recharge potential.
