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Abstract. The estimation of extreme floods is associated with high uncertainty, in part
due to the limited length of streamflow records. Traditionally, statistical
flood frequency analysis and an event-based model (PQRUT) using a single
design storm have been applied in Norway. We here propose a stochastic PQRUT
model, as an extension of the standard application of the event-based PQRUT
model, by considering different combinations of initial conditions, rainfall
and snowmelt, from which a distribution of flood peaks can be constructed.
The stochastic PQRUT was applied for 20 small- and medium-sized catchments in
Norway and the results give good fits to observed peak-over-threshold (POT) series.
A sensitivity analysis of the method indicates (a) that the soil saturation level
is less important than the rainfall input and the parameters of the PQRUT
model for flood peaks with return periods higher than 100 years and (b) that
excluding the snow routine can change the seasonality of the flood peaks.
Estimates for the 100- and 1000-year return level based on the stochastic
PQRUT model are compared with results for (a) statistical frequency analysis
and (b) a standard implementation of the event-based PQRUT method. The
differences in flood estimates between the stochastic PQRUT and the
statistical flood frequency analysis are within 50 % in most catchments.
However, the differences between the stochastic PQRUT and the standard
implementation of the PQRUT model are much higher, especially in catchments
with a snowmelt flood regime.

Better land stewardship is needed to achieve the Paris Agreement's temperature goal, particularly in the tropics, where greenhouse gas emissions from the destruction of ecosystems are largest, and where the potential for additional land carbon storage is greatest. As countries enhance their nationally determined contributions (NDCs) to the Paris Agreement, confusion persists about the potential contribution of better land stewardship to meeting the Agreement's goal to hold global warming below 2°C. We assess cost-effective tropical country-level potential of natural climate solutions (NCS)-protection, improved management and restoration of ecosystems-to deliver climate mitigation linked with sustainable development goals (SDGs). We identify groups of countries with distinctive NCS portfolios, and we explore factors (governance, financial capacity) influencing the feasibility of unlocking national NCS potential. Cost-effective tropical NCS offers globally significant climate mitigation in the coming decades (6.56 Pg CO2e yr-1 at less than 100 US$ per Mg CO2e). In half of the tropical countries, cost-effective NCS could mitigate over half of national emissions. In more than a quarter of tropical countries, cost-effective NCS potential is greater than national emissions. We identify countries where, with international financing and political will, NCS can cost-effectively deliver the majority of enhanced NDCs while transforming national economies and contributing to SDGs. This article is part of the theme issue 'Climate change and ecosystems: threats, opportunities and solutions'.