More generally, on the time scale of centuries, marine transgression may cause rapid salinization of entire aquifers.In Western Europe Holocene transgressions of a few thousands of years have brought salt water of corresponding age to a depth of over 200 m.Nevertheless, at many places all around the world fresh and brackish waters have been found on the continental shelves.Numerical modeling by Post and Simmons illustrates how low-permeability lenses protect fresh water from mixing with downward invading overlying saline ocean waters with higher density.Van Duijn et al.gave a general, modern stability analysis of such density stratified flows below a ponded surface.Saltwater intrusion by tides in the mouths of rivers—The Zuiderzee Works and Delta Plan stopped salinization from tidal motion in the North.In the Southwestern Delta, tidal motion was only partly eliminated and no major freshwater reservoirs are available, like the Lakes IJssel and Marken for the northern provinces.Instead, fresh water supply in the southwest comes more directly from diversions of water from the major rivers.In the 20th century the quality of the Rhine water gradually deteriorated, until a series of international treaties brought improvement.The river water quality was further reduced by an inward directed flow of high-density saline water underneath the outward directed flow of lighter runoff water.Traditionally, the tides had free play and salinized the river water far inland,vertical farm particularly in periods of low river flows.As a result of this salinization, in the 1970s the surface water in the important West land greenhouse district between Rotterdam and the Hague was hardly suitable for use as irrigation water.
The growers themselves made it even worse using drainage return flows, resulting from high leaching fractions combined with high application of fertilizers.The RAND corporation did a policy analysis of water management for the Netherlands , balancing engineering ambitions and agricultural interests, specifically regarding the desired irrigation water quality for use in greenhouse horticulture.The Delta Works have provided some relief from saltwater intrusion in river mouths; however, conflicting agricultural and environmental interests continue to dominate the discussion about seawater blockage as related to the desire to maintain brackish aquatic ecosystems.Saltwater intrusion by inward flow of water to land below sea-level—Fig.32 shows the depth of the brackish-fresh interface in the coastal regions of the Netherlands.Similar maps are available for the coastal region of Belgium.Because fresh water is floating on top of saline groundwater in the dunes area along the west coast, saline intrusion is strongest in the North and Southwest, where coastal dunes are absent.At numerous locations in the dunes, fresh dune water is pumped as a source for preparing drinking water for the western part of the country, where the groundwater is too saline because of continued saltwater intrusion.For example, a dune area of 3400 ha along the western coast supplies fresh drinking water to Amsterdam, already since 1853.To keep the floating bodies of fresh water in the dunes intact, the freshwater pumping is compensated for by excess rainfall and infiltration of river water, partly after having been stored in the Lakes IJssel and Marken.Fresh water floating on top of salt water in agricultural fields—Recently fresh water lenses floating on top of saline groundwater have been fully recognized as being of great importance, not only in the dunes, but also in farmer fields along coastal regions where upward seepage of saline groundwater occurs.
These freshwater lenses can come from rain, melted snow, and increasingly also from irrigation of agricultural lands.Eeman et al.made a detailed analysis of the thickness of a freshwater lens and the transition zone between this lens and the up welling saline water.Starting from a fully saline condition between drains or ditches and assuming constant rates of saltwater up welling and freshwater infiltration, they showed that a freshwater lens will grow until it reaches a maximum size.Moreover, they concluded that the fresh/saline ratio of the drainage water will change from zero to the infiltration/upward seepage ratio.However, as shown by others , seasonal variations of infiltration and plant root withdrawal of fresh water will cause temporal fluctuations of the thickness of the lenses and the fresh-saline ratio of the drainage water.Salt tolerance in a generally humid and cool climate—Most salt tolerance data for field crops and flower species date from before 2000 and were reviewed by Van Bakel et al.and Stuyt et al..The latter compilation in Dutch is the most complete, providing salt tolerance thresholds for35 individual crops or groups of crops.Salt tolerance data for greenhouse horticultural crops were brought together by Sonneveld and Sonneveld and Voogt,and included interactions between plant nutrition and salinity.In the last decade, salt tolerance tests have been carried out at Salt Farm Texel.The 160 m2 experimental plots were irrigated, using eight replications of seven different salt concentrations, obtained by mixing saline seawater with fresh water.Because of the high hydraulic conductivity of the soil, it was possible to maintain the desired concentration throughout the root zone, irrespective of the weather in the growing season.Salt tolerance was tested for six crops: potato , carrot , onion , lettuce , cabbage , and barley.The data were analyzed using the Maas and Hoffman and Van Genuchten and Gupta models.
An alternative model based on the Dalton-Fiscus model for simultaneous uptake of water and solutes was explored by Van Ieperen.Salinization in the countries around the North Sea—In principle, the lowland coastal regions of Belgium, Germany, the Netherlands, Sweden, and the United Kingdom face similar threats from salinity as in the Netherlands.For example, there was widespread flooding of farmland along the UK east coast during the Southern North Sea storm of December 5, 2013.Due to different economic and political priorities, the responses to such events have varied.The Netherlands was saved potential disastrous flooding in 2013, thanks to the Delta Plan response to the 1953 Storm Flood.Gould et al.analyzed the impact of coastal flooding on agriculture in Lincolnshire, UK.They noted that flood risk assessments typically emphasize the economic consequences of coastal flooding on urban areas and national infrastructure and tend to omit the long-term impact of salinization of agricultural land.Considering this long-term salinization, they calculated financial losses ranging from £1366/ha to £5526/ha per inundation, which would be reduced by between 35% up to 85% by post-flood switching to more salt-tolerant crops.Egyptians have practiced irrigated agriculture for about 5000 years in the Nile River valley, using basin irrigation dependent on the rise and fall of flows in the Nile river.Since 3000BCE, the Egyptians used to construct earthen banks to form flood basins of various sizes, filled with the Nile water to saturate soils for crop production.Egyptian irrigated agriculture has been sustainable for thousands of years,nft vertical farming in contrast to other civilizations in Mesopotamia.Reasons were provided by Hillel , pointing to the annual natural flooding that deposited nutrient-rich soil material,annual cycles of rising and falling of the Nile river that created fluctuations of the groundwater table and yearly flushing of salts of its narrow irrigated flood plains, and the annual inundations that occurred in the late summer and early fall, after the spring growing season.With the construction of the Aswan High Dam, most of the land was converted to perennial irrigation and the irrigated area increased from 2.8 to 4.1Mha.The year-around irrigation and lack of leaching by annual pulsing of the Nile river triggered soil salinization.More than 80% of Egypt’s Nile water share is used in agriculture.Water- saving in agriculture is a major challenge because annual per capita water availability in Egypt is expected to decrease to 560m3 from a current level of 950m3.The salts of the Nile basin are either of intrinsic origin, sea water intrusion or from irrigation with saline groundwater.Since the climate of Egypt is characterized as arid with annual rainfall ranging from 5 to 200mm compared to evaporation rates of 1500–2400mm, crop production is not possible in most parts of Egypt without irrigation.Salinity problems in the irrigated areas are widespread and about 1 million ha are already affected.At present only 5.4% of the land resources in Egypt is of excellent quality, while about 42% is relatively poor due to salinity and sodicity problems.Soils in the Nile valley and the Delta are Vertisols, characterized by substantial expansion by wetting and shrinking by drying.In Egypt, productive lands are finite and irreplaceable and thus should be carefully managed and protected against all forms of degradation.Other countries of the Nile basin also have salinity problems.Kenya has about 5Mha of salt-affected lands.In Tanzania, about 30% area is characterized by poor drainage and soil salinity problems.The soil salinity problems in countries such as DR Congo, Uganda, Burundi, and Rwanda are less prevalent however soils are low in fertility.The salt-affected lands in South Sudan and Sudan are in the White Nile irrigation schemes.This area has hardly been utilized for agricultural production despite having great potential due to the availability of water from Nile.In other parts of South Sudan, low soil fertility and lack of good quality seeds for crops and forages are the major bottlenecks in the development of agriculture.
Ethiopia stands first in Africa in the extent of salt-affected soils with an estimated 11Mha of land exposed to salinity.This corresponds to 9% of the total land area and 13% of the irrigated area of the country.These soils are concentrated in the Rift Valley, Wabi Shebelle River Basin, the Denakil Plains and other lowlands and valleys of the country, where 9% of the population lives.Currently, soil salinity is recognized as the most critical problem in the lowlands of the country resulting in reduced crop yields, low farm incomes and increased poverty.The insufficient drainage facilities, poor-quality groundwater for irrigation and inadequate on-farm water management practices are usually held responsible for the increasing salinity problems.Despite the widespread occurrence of salt-affected soils, Ethiopia does not have an accurate data base on the extent, distribution, and causes of salinity development.Most of the saline soils are concentrated in the plain lands of the Rift Valley System, Somali lowlands in the Wabi Shebelle River Basin, the Denakil Plains and various other lowlands and valley bottoms throughout the country.The introduction of large-scale irrigation schemes without the installation of appropriate drainage systems have also resulted in the rapid expansion of soil salinity and sodicity problems in the lower Wabi Shebelle basin of Gode.The distribution of surface salinity in the four largest regions of Ethiopia is given in Table 5.Sudan has built four dams on the Nile during the last century to provide irrigation water to an additional 18,000 km2 of land.This has made Sudan the second most extensive user of the Nile river water, after Egypt.Despite these arrangements, Sudan has not achieved full production potential due to lack of water infrastructure for equitable water distribution among farmers, lack of farm inputs and low soil fertility conditions.In Egypt, about 85% of the available water resources are consumed by the agriculture sector.The completion of Aswan dam increased the intensity of irrigation, which created water logging problems in many parts contributing to the pollution of land and water resources.In Egypt, surface and subsurface drainage systems have been installed to control rising water tables and soil salinity.Besides, crop-based management is used to combat soil salinization.Farmers were encouraged to use agricultural drainage water to irrigate crops thereby reducing disposal problems.However, the unregulated application of drainage water for irrigation has reduced crop yields and polluted soil and water resources.In addition to agricultural chemical residues and salts, drainage waters include treated and untreated domestic wastewater.The use of organic amendments and the mixed application of farmyard manure and gypsum was useful in reducing soil salinity and sodicity.Recently, phytoremediation or plant-based reclamation has been introduced in Sudan, for example to reduce soil sodicity instead of using gypsum.In the absence of surface and subsurface drainage systems, farmers in Ethiopia continue to manage salt-affected soils by adopting traditional salt management solutions.These include:direct leaching of salts,planting salt-tolerant crops,domestication of native wild halophytes for agropastoral systems,phytoremediation,chemical amelioration, and the use of organic amendments such as animal compost.Farmers have also used various drainage designs, allowing salts to settle before its reuse for irrigation water.However, all such practices have failed to mitigate salinity problems in the long-term.Hence crop yields continue to decline, resulting in reduced farm incomes, food shortage and increased poverty.Many of the smallholder farmers are also working as daily laborers, causing unprecedented farmer migration to nearby urban areas and exacerbating prevalent problems of urban unemployment.The increasing demand for food for the rising population in Egypt , the country is trying to expand its irrigated agricultural area.