# Controlled Drainage Effectiveness in Reducing Nutrient Outflow in Light of Climate Changes
## 1 Introduction  
One of the fundamental components of protecting surface water in most European Union countries is the conducting of a realistic assessment of the quantity of nitrogen compounds and other nutrients that are released from agricultural catchments [1; 2; 3]. This evaluation is critical because it determines the fate of the water, particularly when it is at a high risk of eutrophication, such as in the case of the Baltic Sea [4]. Its watershed covers 14 countries, including Poland, which is situated on the coastline and encompasses approximately 18% of the area. Nutrient loads originating from Poland are carried by the Vistula and Oder rivers, as well as the nine rivers of the Przymorze, contributing an estimated 30% of the total nitrogen load introduced into the basin [5]. This is mainly due to the fact that 59.8% of Poland's land is used for agriculture, with 72.4% of it being arable land where crops are accompanied by the intensive application of fertilizers, particularly mineral or chemical nitrogen fertilizers [6]. Considering the additional intensive livestock breeding in Poland, it is assumed that agriculture is the primary economic sector responsible for introducing the largest amount of nutrients into surface waters [7]. The intensification of land use and changes in land and water management in agriculturally used areas necessitates the exploration of new adaptive solutions to realize the potential of artificial agricultural subsurface drainage systems [8; 9; 10; 11]. Presently, depending on the region, they are primarily used for two purposes: to enhance and facilitate the management of soil water associated with salinity and waterlogging and to unilaterally remove excess water, thereby improving soil aeration and field access [12]. As such, tile drainage has been identified as the primary pathway for delivering nutrients and other pollutants to surface waters, which is a significant factor given that an estimated 34% of total agricultural land in northwest Europe is artificially drained. Consequently, reducing the leaching of pollutants from agricultural land into drainage systems could assist in mitigating pollution from agriculture [13]. Therefore, reducing losses from the primary sources of nutrients and their mobilization by enhancing nutrient use efficiency is necessary, and the field scale appears to be an appropriate level to assess this progress [14]. One solution to this issue is CD, which is utilized to decrease the amount of drainage outflow at the scale of the drainage facility, thus reducing the quantity of N and P loads discharged. Annual reported reductions in discharge have ranged from 7% to 68%, while reductions in NO\({}_{3}\)-N loadings have ranged from \(-\)8% to 63% in the USA. However, these studies analyzed the effects of CD on P loading and have reported mixed results, with changes in total P (TP) loading ranging from no effect to 77% reductions [15; 16; 17]. [PERSON] et al. [18] demonstrated that CD led to reductions in drainage volume by 28%, 33%, and 52% under wet, normal, and dry precipitation scenarios, respectively. Additionally, [PERSON] et al. [19] showed a meta-analysis that revealed that CD reduced subsurface drainage volume by approximately 30% for dryland crops. Studies conducted in European countries indicated varying degrees of reduction in drainage volume, ranging from 37% in Denmark to 95% in Sweden; moreover, reductions in nitrate-N were observed, from 36% in Denmark to 100% in Italy. Regarding TP reduction, the percentage of reduction varied, with values ranging from 43% to a range of 56-95% [20; 21; 22]. Taking into account the expected effects of climate change and local physical, geographical, and climatic conditions, environmental and technical solutions are being developed using drainage mitigation measures [23]. These solutions should be able to accommodate future climate changes and agricultural land use, with the aim of achieving crop production goals while minimizing environmental impacts.