Hawliau Cyffredinol / General rightsCopyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.• Users may download and print one copy of any publication from the public portal for the purpose of private study or research.• You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal ? Take down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

The summer of 2018 saw a combination of drought and heat concentrated over northern Europe. The conditions had far-reaching economic and ecological impacts, with spring and summer dryness affecting crops and natural vegetation, leading to increased tree and forest mortality, and unprecedented wildfires in Sweden (Albergel et al., 2019;Rösner et al., 2019). The summer of 2018 was among the warmest, sunniest, and driest on record in the UK (Kendon et al., 2019). Figure 1 quantifies the drought and heatwave, showing the fraction of the 122 days spanning May through August 2018 that lie within the indicated tails of maximum 2 m air temperature anomalies and surface (top 7 cm) volumetric soil water content (VWC), based on the European Centre for Medium-range Weather Forecasts (ECMWF) fifth reanalysis (ERA5) from 1979 to 2018 (Hersbach et al., 2020). One would expect by chance a value of 0.05 at any location in the maximum temperature plot, and 0.25 for VWC. There is strong spatial correspondence between the two panels, but the core

The summer of 2018 saw a combination of drought and heat concentrated over northern Europe. The conditions had far-reaching economic and ecological impacts, with spring and summer dryness affecting crops and natural vegetation, leading to increased tree and forest mortality, and unprecedented wildfires in Sweden (Albergel et al., 2019;Rösner et al., 2019). The summer of 2018 was among the warmest, sunniest, and driest on record in the UK (Kendon et al., 2019). Figure 1 quantifies the drought and heatwave, showing the fraction of the 122 days spanning May through August 2018 that lie within the indicated tails of maximum 2 m air temperature anomalies and surface (top 7 cm) volumetric soil water content (VWC), based on the European Centre for Medium-range Weather Forecasts (ECMWF) fifth reanalysis (ERA5) from 1979 to 2018 (Hersbach et al., 2020). One would expect by chance a value of 0.05 at any location in the maximum temperature plot, and 0.25 for VWC. There is strong spatial correspondence between the two panels, but the core

Abstract. The COSMOS-UK observation network has been providing field-scale soil moisture and hydrometeorological measurements across the UK since 2013. At the time of publication a total of 51 COSMOS-UK sites have been established, each delivering high-temporal resolution data in near-real time. Each site utilizes a cosmic-ray neutron sensor, which counts epithermal neutrons at the land surface. These measurements are used to derive field-scale near-surface soil water content, which can provide unique insight for science, industry, and agriculture by filling a scale gap between localized point soil moisture and large-scale satellite soil moisture datasets. Additional soil physics and meteorological measurements are made by the COSMOS-UK network including precipitation, air temperature, relative humidity, barometric pressure, soil heat flux, wind speed and direction, and components of incoming and outgoing radiation. These near-real-time observational data can be used to improve the performance of hydrological models, validate remote sensing products, improve hydro-meteorological forecasting, and underpin applications across a range of other scientific fields. The most recent version of the COSMOS-UK dataset is publically available at https://doi.org/10.5285/b5c190e4-e35d-40ea-8fbe-598da03a1185 (Stanley et al., 2021).

Abstract. Soil moisture predictions from land surface models are important in hydrological, ecological and meteorological applications. In recent years the availability of wide-area soil-moisture measurements has increased, but few studies have combined model-based soil moisture predictions with in-situ observations beyond the point scale. Here we show that we can markedly improve soil moisture estimates from the JULES land surface model using field scale observations and data assimilation techniques. Rather than directly updating soil moisture estimates towards observed values, we optimize constants in the underlying pedotransfer functions, which relate soil texture to JULES soil physics parameters. In this way we generate a single set of newly calibrated pedotransfer functions based on observations from a number of UK sites with different soil textures. We demonstrate that calibrating a pedotransfer function in this way can improve the performance of land surface models, leading to the potential for better flood, drought and climate projections.