water Article Effect of Bypass-Flow on Leaching of Salts in a Cracking Soil in the Nile Delta Haruyuki Fujimaki * and Hassan Mohamed Fahmy Abd El Baki Arid Land Research Center, Tottori University, Tottori 680-0001, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-857-21-7040; Fax: +81-857-29-6199 Abstract: Salinity is a major threat to the sustainability of irrigated agriculture in arid and semi-arid regions. Leaching is the primary measure for removing excess salts from the root zone, but not all water applied to the soil surface contributes to the removal of salts. In clayey soils, bypass flow along cracks can occur without being mixed with saline pore water in the matrix. To present a field dataset to quantitatively evaluate the contribution of bypass flow to the leaching of salts, soil sampling and monitoring of groundwater and discharge from a tile drain were carried out in farmland having a cracking soil in the Nile Delta. The electrical conductivities of 1:2 extracts were measured to evaluate the salinity of the soil. The first evidence for the occurrence of significant bypass flow through cracks was the salinity of the pore water, which was nearly triple that of the shallow groundwater and outflow from drainage. Second, the difference in root zone salinity before and after paddy rice cultivation was not significant. Third, the gradient of the groundwater table was very small. in spite of the low saturated hydraulic conductivity. Fourth, the salinity of the outflow from the tile drain dropped just after irrigation or rain. These results indicated that bypass flow through cracks played a significant role in the drainage process in the soil, and that nearly half of the water bypasses through cracks in the field with a cracking soil. Keywords: salinity stress; solute transport; preferential flow; drainage Citation: Fujimaki, H.; Abd El Baki, H.M.F. Effect of Bypass-Flow on Leaching of Salts in a Cracking Soil in the Nile Delta. Water 2021, 13, 993. 1. Introduction https://doi.org/10.3390/w13070993 Since the dawn of civilization, salinity has been a major threat to the sustainability of irrigated agriculture in arid and semi-arid regions. For example, Alexakis et al. [1] assessed Academic Editor: Francesco Fiorillo the soil salinization in western Greece and found that the soil salinity is mainly controlled by seawater intrusion or other factors such as the application of salt-rich drainage water [2]. To Received: 5 March 2021 remove excess salts from the root zone to mitigate the salinity hazard, more water than that Accepted: 2 April 2021 Published: 4 April 2021 required to meet crop evapotranspiration is periodically applied [3]. Such an intentional “over-irrigation” is called leaching, which has been the primary measure and widely Publisher’s Note: MDPI stays neutral practiced for salinity control. A guideline for calculating leaching requirements, how much with regard to jurisdictional claims in water should be applied for leaching, was presented by Food and Agriculture Organization published maps and institutional affil- (FAO) irrigation and drainage paper 29. However, not all water applied to the soil surface iations. contributes to removing salts. In clayey soils, bypass flow along cracks formed with drying and shrinking can occur without being mixed with saline pore water in the matrix. FAO paper 29 recommends breaking cracks by tillage and applying water intermittently to reduce bypass flow. However, no procedure to determine the additional water required to compensate the bypass flow has been presented. Recently, Minhass et al. [4] reviewed Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. recent developments in salinity management studies but did not refer to the matter of This article is an open access article bypass flow. The lack of field data and quantitative evaluation of the contribution of bypass distributed under the terms and flow may account for the inadequate guidelines and undervaluing of this problem. Surface conditions of the Creative Commons irrigation is still dominant in developing countries, and leaching is usually carried out Attribution (CC BY) license (https:// under continuous ponding [5]. creativecommons.org/licenses/by/ Oster et al. [6] compared three methods of leaching: continuous ponding, intermittent 4.0/). ponding, and sprinkler, and noted that the lower leaching efficiency of continuous ponding Water 2021, 13, 993. https://doi.org/10.3390/w13070993 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 12 Water 2021, 13, 993 Oster et al. [6] compared three methods of leaching: continuous ponding, intermit- 2 of 11 tent ponding, and sprinkler, and noted that the lower leaching efficiency of continuous ponding is attributable to bypass flow in cracks and macropores. Moreno et al. [7] re- ported a 10 times largeris attributable apparent to intake bypass rate flow for in ground cracks and with macropores. cracks than Moreno a matrix et al.in [Las7] reported a 10 times Marismas, Spain, largerand recommended apparent intake a shorter rate for irrigation ground with interval cracks and than lower a matrix input in each Las Marismas, Spain, time. Corwin et al.and [8] recommendedcarried out a lysimeter a shorter experiment irrigation interval and noted and lowerthat the input effect each of time.by- Corwin et al. [8] pass flow was smallcarried because out it aoccurs lysimeter only experimentin the top 30 and cm, notedalthough that most the effectwater ofis taken bypass flow was small up by the annual irrigatedbecause itcrop occurss from only the in top the 30 top cm. 30 Still, cm, althoughthere is a mostlack of water fieldis data taken to up by the annual quantitatively evaluateirrigated how crops much from additional the top water 30 cm. is needed Still, there to attain is a the lack target of field salinity data to quantitatively in the root zone. evaluate how much additional water is needed to attain the target salinity in the root zone. The objective of thisThe study objective was, therefore, of this study to present was, therefore, a field dataset to present from a cracking field dataset soil from cracking soil in the Nile Delta toin quantitatively the Nile Delta evaluate to quantitatively the contribution evaluate of the bypass contribution flow in the of bypassleaching flow in the leaching of salts. of salts. 2. Materials and Methods2. Materials and Methods ◦ The field observationThe fieldwas observationcarried out was in carriedan experimental out in an experimental farmland in farmland Sakha in(31° Sakha (31 05’53.9” N, ◦ 05’53.9” N, 30°55’19.130 55’19.1”” E), Egypt, E), Egypt,which was which managed was managed by the Agricultural by the Agricultural Research Research Cen- Center of Egypt ter of Egypt (Figure(Figure 1). The1). field The fieldwas under was under a series a series of cropping of cropping experiment experimentss to evaluate to evaluate actual evap- actual evapotranspirationotranspiration rates from rates a fromsurface a surface irrigated irrigated field using field the using eddy the correlation eddy correlation method [9]. method [9]. CultivatedCultivated crops cropsand periods and periods are listed are listedin Table in Table 1. All1 .crops All crops were were irrigated irrigated with surface with surface irrigation.irrigation. The electrical The electrical conductivity conductivity of the of irrigation the irrigation water water fluctuated fluctuated be- between 0.4 and −1 tween 0.4 and 0.5 dS0.5 m dS−1.m . Figure 1. LocationFigure 1. ofLocation the study of thesite. study Area site.painted Area green painted represents green represents farmland farmlandand cities. and cities. Table 1. Cultivation recordThe of the farmland farmland. had tile drainage pipes, whose inner diameter, depth, and length were Period approximately 0.072, 1.5,Crop and 50 m, respectively.Irrigation No envelope Method material was used for the tile drainage pipes. Nine observation wells were installed to monitor the groundwater 2011/2012 winter Wheat Basin level and EC at an equal spacing of 6.25 m along a line perpendicular to the tile drains, 2012 summer Rice Basin as illustrated in Figure2. Groundwater level was measured using a water level meter 2012/2013 winter(Yamayo, WL10M, Tokyo,Wheat Japan). Basin Water 2021, 13, x FOR PEER REVIEW 3 of 12 2013 summer Maize Furrow Water 2021, 13, 993 3 of 11 2013/2014 winter Sugarbeet Furrow 2014 summer Rice Basin The farmlandTable had 1. Cultivation tile drainage record pipes of the, whose farmland. inner diameter, depth, and length were approximately 0.072, 1.5, and 50 m, respectively. No envelope material was used for the Period Crop Irrigation Method tile drainage pipes. Nine observation wells were installed to monitor the groundwater level and EC at an equal2011/2012 spacing winter of 6.25 m along a line perpendicular Wheat to the tile drains Basin, as 2012 summer Rice Basin illustrated in Figure 2012/20132. Groundwater winter level was measured Wheat using a water level meter (Ya- Basin mayo, WL10M, Tokyo,2013 Japan summer). Maize Furrow On 18 April 2012,2013/2014 23 April winter 2014, and 20 October 2014 Sugarbeet soil samples were taken to Furrow ob- tain soil salinity profiles.2014 The summer first sampling was carried Rice out at two randomly selected Basin locations, while the others were taken at certain intervals from the lateral drains. Figure 2. Structure of lateral and collector drains. Monitoring of discharge was carried out in man- Figure 2. Structure of lateral and collector drains. Monitoring of discharge was carried out in manhole 1. Orange lines hole 1. Orange lines represent roads. represent roads. The electrical conductivityOn 18 April 2012, (EC) 23of April(soil-to 2014,-water and ratio 20 October) 1:2 suspension 2014 soil was samples measured, were taken to obtain and pore watersoil EC salinity was calculated profiles.