Modelling shadow and cover effects on manure temperature

Manure temperature is crucial to estimate methane emissions from manure management systems. Since measured data of manure temperature is rare, mechanistic models are used to simulate manure temperature. However, previous mechanistic models did not consider the shadow and cover effects that reduce solar radiation and impact evaporation, which results in a decrease or increase of manure temperature. The objectives of this study were (1) to develop modules for simulating the effect of shadow and tank cover effects on manure temperature, and (2) to analyze the sensitivity of the manure temperature and cumulative radiation input to different latitudes and designs of manure tanks. Modules were developed to a) estimate the shadow effect caused by the tank wall which considers the angle of direct solar radiation and the height of the wall above the manure surface, b) to estimate the effects of a polyester cover, that reduces direct solar radiation and evaporation, and c) to include the effects of snow cover in winter that increase surface albedo and enthalpy of fusion while melting. Independent validation assessment was conducted using manure temperatures at 0.5m, 1.5m, and 2.5m depth in two manure tanks (20 m in diameter, with and without polyester cover), measured in Sweden from 2020 June to May 2021. In the tank without a cover, the average annual and summer manure temperatures were 8.4°C and 15.4°C, respectively, while the modified model estimated these temperatures to be 7.9°C and 16.4°C. In the tank with a polyester cover, the average annual and summer manure temperatures were 9.5°C and 16.1°C, respectively while the modified model predicted values of 9.4°C and 16.2°C. The modified model showed high model accuracy (R2 = 0.94 and 0.93, RMSE = 1.8°C and 1.3°C, and d index = 0.97 and 0.98, for the tank without and with a polyester cover respectively). Sensitivity analysis suggests that smaller diameter but deeper tanks should be used when feasible to reduce manure temperature in high latitude areas. The inclusion of shadow and cover effects in the manure temperature model improved model performance. These improvements are important as a foundation for simulating methane emissions using simpler (i.e. 2019 IPCC Refinement) or more complex modelling approaches.