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In coastal zones with saline groundwater, fresh groundwater lenses may form due to infiltration of rain water. The thickness of both the lens and the mixing zone, determines fresh water availability for plant growth. Due to recharge variation, the thickness of the lens and the mixing zone are not constant, which may adversely affect agricultural and natural vegetation if saline water reaches the root zone during the growing season. In this paper, we study the response of thin lenses and their mixing zone to variation of recharge. The recharge is varied using sinusoids with a range of amplitudes and frequencies. We vary lens characteristics by varying the Rayleigh number and Mass flux ratio of saline and fresh water, as these dominantly influence the thickness of thin lenses and their mixing zone. Numerical results show a linear relation between the normalised lens volume and the main lens and recharge characteristics, enabling an empirical approximation of the variation of lens thickness. Increase of the recharge amplitude causes increase and the increase of recharge frequency causes a decrease in the variation of lens thickness. The average lens thickness is not significantly influenced by these variations in recharge, contrary to the mixing zone thickness. The mixing zone thickness is compared to that of a Fickian mixing regime. A simple relation between the travelled distance of the centre of the mixing zone position due to variations in recharge and the mixing zone thickness is shown to be valid for both a sinusoidal recharge variation and actual records of daily recharge data. Starting from a step response function, convolution can be used to determine the effect of variable recharge in time. For a sinusoidal curve, we can determine delay of lens movement compared to the recharge curve as well as the lens amplitude, derived from the convolution integral. Together the proposed equations provide us with a first order approximation of lens characteristics using basic lens and recharge parameters without the use of numerical models. This enables the assessment of the vulnerability of any thin fresh water lens on saline, upward seeping groundwater to salinity stress in the root zone.

Vietnam is among the top five countries that will be most affected by sea level rise. This study aimed to assess the subsequent impacts of flooding and salinity intrusion on, and to evaluate suitable adaptation strategies for the Mekong Delta's pangasius farming sector. Water level rise and salt water intrusion for three sea level rise (SLR) scenarios (i.e. +30cm, +50cm and +75cm) were simulated by using the MIKE11 model. The results showed that at SLR+50, the 3m flood level would spread downstream and threaten farms located in upstream and midstream regions. Rising salinity for SLR+75 would reduce the appropriate time-window for the culture in coastal areas. A Chi-Square test and a logit regression model were employed to examine factors which influence pangasius farmers’ perception of and adaptation to climate change impacts. Less than half of the respondents were concerned about climate change and actively sought suitable adaptation measures to alleviate its impacts. The adaptive capacity of pangasius farmers can be improved by increasing the information on climate change and introducing early warning systems. The technical efficiency (TE) of randomly sampled pangasius farms was estimated using Data Envelopment Analysis, and factors affecting technical and scale efficiency were examined with bootstrap truncated regression. The mean TE score assuming constant return to scale was 0.66, and under variable return to scale it was 0.84. TE of downstream farms was higher compared to the upstream and midstream farms due to lower energy costs and stocking once a year at a lower density, but these reduced the scale efficiency of farms affected by salinity intrusion. Upstream and midstream farms needed to pump water and stocked at least three times in two years. Regression analysis showed a positive effect on TE of the farmer’s education level, and of having experienced climate change impact through flooding or salinity intrusion in the past. Using a decision tree framework, this study analyzed possible options for adapting pangasius farming to the projected climate-change impacts. Options to adapt to salinity intrusion are: modify the pangasius farming practice by using e.g. water recirculation systems, stock other species, or stock saline-tolerant pangasius with support from research and extension. A breeding program for saline-tolerant striped catfish requires long-term investments (0.4 % of the present production costs). To adapt to worse flooding, pangasius farms not located within the upgraded government dyke-protected areas could raise the height of the dyke around their pangasius farm, which would increase the total variable costs per ha for one harvest by about 0.34% in the upstream and midstream regions, and by 0.25% in the downstream region.

Vietnam is among the top five countries that will be most affected by sea level rise. This study aimed to assess the subsequent impacts of flooding and salinity intrusion on, and to evaluate suitable adaptation strategies for the Mekong Delta's pangasius farming sector. Water level rise and salt water intrusion for three sea level rise (SLR) scenarios (i.e. +30cm, +50cm and +75cm) were simulated by using the MIKE11 model. The results showed that at SLR+50, the 3m flood level would spread downstream and threaten farms located in upstream and midstream regions. Rising salinity for SLR+75 would reduce the appropriate time-window for the culture in coastal areas. A Chi-Square test and a logit regression model were employed to examine factors which influence pangasius farmers’ perception of and adaptation to climate change impacts. Less than half of the respondents were concerned about climate change and actively sought suitable adaptation measures to alleviate its impacts. The adaptive capacity of pangasius farmers can be improved by increasing the information on climate change and introducing early warning systems. The technical efficiency (TE) of randomly sampled pangasius farms was estimated using Data Envelopment Analysis, and factors affecting technical and scale efficiency were examined with bootstrap truncated regression. The mean TE score assuming constant return to scale was 0.66, and under variable return to scale it was 0.84. TE of downstream farms was higher compared to the upstream and midstream farms due to lower energy costs and stocking once a year at a lower density, but these reduced the scale efficiency of farms affected by salinity intrusion. Upstream and midstream farms needed to pump water and stocked at least three times in two years. Regression analysis showed a positive effect on TE of the farmer’s education level, and of having experienced climate change impact through flooding or salinity intrusion in the past. Using a decision tree framework, this study analyzed possible options for adapting pangasius farming to the projected climate-change impacts. Options to adapt to salinity intrusion are: modify the pangasius farming practice by using e.g. water recirculation systems, stock other species, or stock saline-tolerant pangasius with support from research and extension. A breeding program for saline-tolerant striped catfish requires long-term investments (0.4 % of the present production costs). To adapt to worse flooding, pangasius farms not located within the upgraded government dyke-protected areas could raise the height of the dyke around their pangasius farm, which would increase the total variable costs per ha for one harvest by about 0.34% in the upstream and midstream regions, and by 0.25% in the downstream region.

Het veiligstellen van de toekomstige zoetwatervoorziening in het Hoogheemraadschap van Rijnland kan in beginsel op verschillende manieren worden geeffectueerd. Elke maatregel die hiertoe wordt overwogen beïnvloedt het chloridegehalte van het polderwater en daarmee de kwaliteit van beregeningswater voor de landbouw en de ecologische kwaliteit. Een aanzienlijke reeks maatregelen is geëvalueerd met behulp van €ureyopener 1.0, een in MS Excel ontwikkeld analyse-instrument, bedoeld voor interactief gebruik. De handelingsruimte die beschikbaar is om anders om te gaan met zoet water zijn geanalyseerd en in beeld gebracht, uitgedrukt in effect op de watervraag, zoutschade aan landbouwgewassen en effecten op de natuur.

Het veiligstellen van de toekomstige zoetwatervoorziening in het Hoogheemraadschap van Rijnland kan in beginsel op verschillende manieren worden geeffectueerd. Elke maatregel die hiertoe wordt overwogen beïnvloedt het chloridegehalte van het polderwater en daarmee de kwaliteit van beregeningswater voor de landbouw en de ecologische kwaliteit. Een aanzienlijke reeks maatregelen is geëvalueerd met behulp van €ureyopener 1.0, een in MS Excel ontwikkeld analyse-instrument, bedoeld voor interactief gebruik. De handelingsruimte die beschikbaar is om anders om te gaan met zoet water zijn geanalyseerd en in beeld gebracht, uitgedrukt in effect op de watervraag, zoutschade aan landbouwgewassen en effecten op de natuur.

Het doel van het deelproject ‘Sociaal Economisch Spoor’ is om de effecten van verzilting in sociaal economische zin op landbouw inzichtelijk te maken voor Noord-Nederland. Hierbij wordt onderscheid gemaakt in directe gevolgen voor de interne agrarische bedrijfsvoering en externe factoren. Inzicht in interne bedrijfsvoering omvat zaken als het beheersen van waterkwaliteit op perceelsniveau door waterbeheersmaatregelen. Belangrijke externe factoren zijn vestigingsklimaat, markt, regelgeving en kredietverlening.

Kennis voor Klimaat, Rijkswaterstaat en STOWA/Deltaproof hielden op 31 mei en 1 juni 2012 het tweedaags symposium ‘Verzilting & waterbeheer: uitdagingen voor beleid, kennis en beheer’ over de toenemende verzilting van ons grond- en oppervlaktewater, die extra wordt aangejaagd door klimaatverandering. In deze Zoet-Zoutkrant o.a. de volgende bijdragen: - Toenemende verzilting vereist mentale omslag bij waterbeheerders en gebruikers; - Omgaan met zout: niet-technische maatregelen : voors en tegen van het beprijzen van zoetwater; - Wat kunnen we doen in het hoofdwatersysteem om zoetwatertekorten en verzilting tegen te gaan? : zoutindringing voorkomen, maar tegen welke prijs?; - Zoutgevoelige natuur en landbouw : flexibeler omgaan met chloridenormen; - Verzilting en waterbeheer, maatregelen in het regionale systeem: het ene waterschap is het andere niet...

Kennis voor Klimaat, Rijkswaterstaat en STOWA/Deltaproof hielden op 31 mei en 1 juni 2012 het tweedaags symposium ‘Verzilting & waterbeheer: uitdagingen voor beleid, kennis en beheer’ over de toenemende verzilting van ons grond- en oppervlaktewater, die extra wordt aangejaagd door klimaatverandering. In deze Zoet-Zoutkrant o.a. de volgende bijdragen: - Toenemende verzilting vereist mentale omslag bij waterbeheerders en gebruikers; - Omgaan met zout: niet-technische maatregelen : voors en tegen van het beprijzen van zoetwater; - Wat kunnen we doen in het hoofdwatersysteem om zoetwatertekorten en verzilting tegen te gaan? : zoutindringing voorkomen, maar tegen welke prijs?; - Zoutgevoelige natuur en landbouw : flexibeler omgaan met chloridenormen; - Verzilting en waterbeheer, maatregelen in het regionale systeem: het ene waterschap is het andere niet...

In Nederland neemt de aandacht voor verzilting toe, vooral vanwege mogelijke gevolgen voor de landbouw en natuur. Om inzicht te krijgen in de situatie voor de landbouw, in het bijzonder voor de melkveehouderij, is een literatuurstudie uitgevoerd. In deze studie is geprobeerd antwoorden te vinden op een aantal vragen, waaronder: (1) waardoor wordt verzilting veroorzaakt?; (2) is verzilting een serieus probleem voor de landbouw?; (3) zijn er regionale verschillen in oorzaken en gevolgen van verzilting?; (4) welk effect zal klimaatverandering op verzilting hebben?; en (5) hoe kan het best met de gevolgen van verzilting omgegaan worden? De ernstigste oorzaken van verzilting, die grootschalig op kunnen treden (vaak via kwel) en kunnen leiden tot hoge chloridegehalten in grond- en oppervlaktewater, zijn de aanwezigheid van fossiel (zout) zeewater in relatief ondiepe bodemlagen (o.a. in de kuststrook), zeewaterindringing langs de zeekust en zeewaterindringing via de Nieuwe Waterweg.

Upward saline groundwater seepage is leading to surface water salinization of low-lying polders in the Netherlands. Identifying measures to reduce the salt content requires a thorough understanding and quantification of the dominant sources water and salt on a daily basis. However, as in most balance studies, there are large uncertainties about the contribution of groundwater seepage. Taking these into account, we applied a probabilistic (GLUE) end-member mixing approach to simulate two years of daily to weekly observations of discharge, salt loads and salt concentrations of water pumped out of an artificially drained polder catchment area. We were then able to assess the contribution of the different sources to the water and salt balance of the polder and the uncertainties in their quantification. Our modelling approach demonstrates the need to distinguish preferential from diffuse seepage. Preferential seepage via boils contributes, on average, 66% to the total salt load and only about 15% to the total water flux into the polder and therefore forms the main salinization pathway. With the model we were able to calculate the effect of future changes on surface water salinity and to assess the uncertainty in our predictions. Furthermore, we analyzed the parameters sensitivity and uncertainty to determine for which parameter the quality of field measurements should be improved to the reduce model input and output uncertainty. High frequency measurements of polder water discharge and weighted concentration at the outlet of the catchment area appear to be essential for obtaining reliable simulations of water and salt fluxes and for allotting these to the different sources.

Kuttanad, the low-lying tract in Kerala State (South-west India), is a region where excess water has caused the agricultural production to remain low. This is even more severe in the potential acid sulphate soils of Kuttanad. Besides the problems inherent to these soils, the region also experiences floods, lack of fresh water and intrusion of saline water from the Arabian Sea. A subsurface drainage system consisting of 10 cm diameter clay tiles, each of 60 cm length, was installed in a pilot area to study the effect of drainage in alleviating the problems faced by these soils. The drains were installed at a depth of about 1 m with spacings of 15 m (replicated 5 times with a drain length of 75 m) and 30 m (replicated twice with a drain length of 100 m) in order determine the drainage parameters from the field data. The paper discusses on the evaluation of drainage parameters from field data obtained from a pilot area where subsurface drainage system was installed and the use these parameters for designing larger drainage systems. This particular study pertains to the polders of Kuttanad where the fields are always surrounded by water bodies. With the collected drain outflow and water table subsidence data, the drainage parameters namely the hydraulic conductivity and the drainable porosity were determined. A study conducted in subsurface drained field has shown that the hydraulic conductivity values were comparatively higher for the topsoil in the experimental area. It varied directly with the mid-spacing water table height for drains close to the outside water body and exponentially for drains away from the outside water body. The transition in the mode of variation took place at a distance of 60 m from the water body. The values also showed a decreasing trend with the distance of the field from the water body for the same mid-spacing water table heights. The average equivalent hydraulic conductivity computed was 0.167 m/d for regions up to 60 m from the water body and 0.055 m/d for regions beyond that. For areas near to outside water bodies, the drainable porosity increased as the mid-spacing water table height decreased. For areas that are away from the water bodies, the drainable porosity decreased with the mid-spacing water table height and became almost constant at lower mid-spacing water table heights. The average equivalent drainable porosity for the flow domain that are away from the water bodies is 0.04.

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Kuttanad, the low-lying tract in Kerala State (South-west India), is a region where excess water has caused the agricultural production to remain low. This is even more severe in the potential acid sulphate soils of Kuttanad. Besides the problems inherent to these soils, the region also experiences floods, lack of fresh water and intrusion of saline water from the Arabian Sea. A subsurface drainage system consisting of 10 cm diameter clay tiles, each of 60 cm length, was installed in a pilot area to study the effect of drainage in alleviating the problems faced by these soils. The drains were installed at a depth of about 1 m with spacings of 15 m (replicated 5 times with a drain length of 75 m) and 30 m (replicated twice with a drain length of 100 m) in order determine the drainage parameters from the field data. The paper discusses on the evaluation of drainage parameters from field data obtained from a pilot area where subsurface drainage system was installed and the use these parameters for designing larger drainage systems. This particular study pertains to the polders of Kuttanad where the fields are always surrounded by water bodies. With the collected drain outflow and water table subsidence data, the drainage parameters namely the hydraulic conductivity and the drainable porosity were determined. A study conducted in subsurface drained field has shown that the hydraulic conductivity values were comparatively higher for the topsoil in the experimental area. It varied directly with the mid-spacing water table height for drains close to the outside water body and exponentially for drains away from the outside water body. The transition in the mode of variation took place at a distance of 60 m from the water body. The values also showed a decreasing trend with the distance of the field from the water body for the same mid-spacing water table heights. The average equivalent hydraulic conductivity computed was 0.167 m/d for regions up to 60 m from the water body and 0.055 m/d for regions beyond that. For areas near to outside water bodies, the drainable porosity increased as the mid-spacing water table height decreased. For areas that are away from the water bodies, the drainable porosity decreased with the mid-spacing water table height and became almost constant at lower mid-spacing water table heights. The average equivalent drainable porosity for the flow domain that are away from the water bodies is 0.04.

Van het noordelijk deel van het herinrichtingsgebied Bergen-Schoorl is het geohydrologische systeem beschreven op basis van abiotisch onderzoek: geologische opbouw en historische ontwikkeling, bodemopbouw, hoogteligging en geomorfologie, hydrologie enhydrochemie. Er zijn vier geohydrologische systemen onderscheiden: Hondsbossche systeem (zout, Noordzee, duinen, Hondsbossche Vaart); Binnenduinrandsysteem (zoet, duinen); Noordhollandsch Kanaalsysteem (vervuild, polders, duinen, Noordhollandsch Kanaal);Weichsysteem (gemengd, polders). Het Weichsysteem is onderverdeeld in vier subsystemen: Slaperdijksubsysteem (zoet-brak-zout); Pettemersubsysteem (brak-zout); Hargersubsysteem (brak-zoet); Groetersubsysteem (zoet-brak, deels vervuild).

Van het noordelijk deel van het herinrichtingsgebied Bergen-Schoorl is het geohydrologische systeem beschreven op basis van abiotisch onderzoek: geologische opbouw en historische ontwikkeling, bodemopbouw, hoogteligging en geomorfologie, hydrologie enhydrochemie. Er zijn vier geohydrologische systemen onderscheiden: Hondsbossche systeem (zout, Noordzee, duinen, Hondsbossche Vaart); Binnenduinrandsysteem (zoet, duinen); Noordhollandsch Kanaalsysteem (vervuild, polders, duinen, Noordhollandsch Kanaal);Weichsysteem (gemengd, polders). Het Weichsysteem is onderverdeeld in vier subsystemen: Slaperdijksubsysteem (zoet-brak-zout); Pettemersubsysteem (brak-zout); Hargersubsysteem (brak-zoet); Groetersubsysteem (zoet-brak, deels vervuild).