Soil warming by electrical underground transmission lines impacts temporal dynamics of soil temperature and moisture

BackgroundThe current transformation of the entire energy system leads to a large‐scale expansion of extra‐high‐voltage underground transmission lines (UTL). Knowledge of the impact on soil temperature and soil moisture dynamics is fundamental for environmental evaluation.AimsWe investigated the impact of an existing 320 kV underground cable in continuous operation on soil temperature and moisture dynamics.MethodsA soil‐monitoring programme was established at four study sites in Western Germany. Data were continuously recorded in soil up to 120 cm depth using soil sensors over a period of 1 year.ResultsSoil warming was in a range of 0.6 K in the topsoil, approx. 1–1.3 K in the rooting zone and 1.7 K in the subsoil at 120 cm depth and was restricted mainly to the immediate vicinity of the cable route. Likewise, the impact on soil moisture dynamics was on average in a range of −1.00 wt.‐% in 0–60 cm depth and −2.45 wt. 2‐% in the subsoil relative to control. Although at a calculated maximum load capacity of 100% in regular operation, soil warming might remain moderate, with 1.5 K in the topsoil, 2.3–3.1 K in the rooting zone and 4.1 K in the subsoil.ConclusionsIt is assumed that the reasons for the low‐to‐moderate influence of the UTL are to be found in the operational cable load (on average 65%), heat loss of cables (approx. 12 W m−1 per cable) and the quality of the imbedding material for the cables.