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Water and salt transport in the irrigated cracking clay soils of the kachhi plains, Pakistan part II. Horizontal transport
SOIL T E C H N O L O G Y
vol. 2, p. 101-106
COMMUNICATION [
WATER A N D SALT TRANSPORT IN THE IRRIGATED CRACKING CLAY SOILS OF THE KACHHI PLAINS, PAKISTAN PART II. HORIZONTAL TRANSPORT A. K a m p h o r s t , W a g e n i n g e n
Summary A description of the vertical transport of water and salts in the irrigated cracking clay soils of the Kachhi Plains of Pakistan (KAMPHORST 1988) is followed by an analysis of the horizontal transport components. During basin irrigation water is transported through the cracks of the topsoil, from oblong erosional microdepressions towards isolated elevations. This leads to the formation of external solonchaks on the elevated parts of the fields. As better levelling of the fields is difficult to achieve, leaching of the salts from the elevations by heavy irrigation applications is the only solution to this salt problem. Under intermittently irrigated crops, however, this is not quite possible, as it leads to water logging in the depressions. Therefore, it is recommended to grow a wet rice crop once in several years.
ISSN 0933-3630 @1989 by CATENA VERLAG, D-3302 Cremlingen-Destedt,W. Germany 0933-3630/89/5011851/US$ 2.00 + 0.25 SOIL TECHNOLOOY--A cooperating Journal of CATENA
1
Introduction
KAMPHORST (1988) showed that the vertical transport of water and salts in fine-textured topsoils of the Kachhi Plains in Pakistan takes place almost exclusively in the cracks. Due to the slow and incomplete closing of these cracks upon wetting of the soils, this "bypass" transport may continue for more than one month under continuous flooding. In deep fine textured soils the internal drainage and salt leaching are limited by the uncracked fine textured subsoils rather than by the cracked fine textured topsoils. In the situation just described the movement of free water through the inter-aggregate cleavages may take place not only in a vertical but also in a horizontal direction, particularly if the subsoil is impervious to water transport. Horizontal bypass flow in gently to moderately sloping black cotton soils in India was already mentioned in a companion paper (KAMPHORST 1988). The piping gullies in such soils are formed when rain showers fall on a dry, cracked surface soil. The water entering the cracks then runoff at the bottom of the cracked topsoil. Downslope the accumulated sub-
102
Kamphorst
surface runoff creates pipes or tunnels by erosion. U p o n their collapse these pipes turn into rills and, finally, gullies (photo 1). Not only in eroding landscapes but also in sedimentary plains were the effects of horizontal bypass flow in cracking clay soils observed. In the uncultivated Kachhi Plains, for instance, sedimentation takes place by "flash floods", which enter the piedmont plain from the hills after rainfall. When the flood water reaches the fine-textured soils it enters the crack system, where it deposits its entire sediment load. In this way the cracks sometimes are filled with sediment almost up to the soil surface (photo 2). VAN H O O R N (1984) reported preferential horizontal saturated transport of rain water towards drain trenches through highly permeable topsoils overlying slowly permeable clay layers in Spain and Portugal. The hig hydraulic conductivity o f the top layers was attributed to a better soil structure of the tilled layer in arable land and of the turf layer in grassland. This paper describes how horizontal transport through cracks takes place during basin irrigation o f the fine-textured Kachhi Plain soils. In that situation it influences not only the water distribution but also the salt patterns in the irrigated fields.
2
Background
Under flood or basin irrigation, water is generally applied to a field by making a breach in the bund of an irrigation channel. Due to this system water enters the field from one side, creating a wetting front that advances via the lowest parts towards the highest parts of the field. If the soil to be irrigated is an initally dry,
cracked clay soil, the water enters the crack system at the wetting front and then runs through the cracks towards the more distant parts of the field, as is shown in fig.1. The existence of such water distribution patterns in the cultivated fields of the heavy piedmont plain soils of the Khirtar Branch Command areas was confirmed by auger observations during the application of irrigation water. It was observed that the horizontal bypass flow took place concentrically from oblong depressions, created by microerosion, towards the higher parts of the fields in between. The lateral bypass flow shown in fig.1 implies that initial wetting of the soil in fact does not take place at the soil surface but at the bottom of the cracked topsoil. Due to this the infiltration of water into the prismatic soil aggregates has a horizontal concentric as well as an upward component. Hence the resident salts are washed towards the centre of the prisms ( K A M P H O R S T & B H A N A S A W Y 1988) and towards the soil surface. The latter component of the salt movement leads to the formation of external solonchaks, i.e., saline soils in which the salt content is highest at the soil surface and decreases with depth. As long as the subsoil below the cracked zone absorbs water, the concentric and upward capillary transport of water inside the prismatic aggregates is accompanied by downward bypass transport in the cracks. In principle this occurs at any point in the field, before as well as after the passage of the wetting front. This downward water transport is, however very inefficient with respect to salt leaching, as the salts are accumulated in the interiors of the prisms by the concentric flow.
SOIL TECHNOLOGY--A cooperating Journal of CATENA
Salt Transport in Cracking Clay Soils
103
N
r-77q77
X\/,
",~
\,,4
\'~/
"v
"",J
\,'
M
dry prismatic surface soil dry blocky subsurfuce soil wef prismafic surfQce soil wef blocky subsurface soil free wafer
Fig. 1: Schematic vertical cross-section, perpendicular to the irrigation front, of an
initially dry field on a cracking clay soil.
Appreciable leaching of salts can take place only where the downward transport lasts for many days after the prisms are wetted. During such a period the salts can return to the exteriors of the aggregates and from there to the bypass water by molecular diffusion ( K A M P H O R S T & BHANASAWY 1988). Long term flooding and hence vertical bypass transport of appreciable duration takes place only in the depressed parts of the fields. For instance, it was observed that inundation lasted for more than 1 week in the depressions of deep fine-textured soils after a heavy irrigation application, while the highest parts of the elevations were often not flooded at all. It is in these situations in particular that one should find external solonchaks on the elevations and non-saline or slighlty saline soils in the depressed parts of the irrigated fields.
3
Materials and methods
A large irrigated wheat field on deep fine textured soils, showing visible patches of
S O I L T E C H N O L O G Y - - A cooperating Joortaal of CATENA
salt damage (see photo 3), was selected to check the model described above. Seventeen auger holes to a depth of 200 cm were made along a straight line passing through several salt patches. At each auger site the level of the soil surface was determined with the aid of an accurate levelling instrument and the crop cover was recorded. From the auger holes soil samples were taken from the depth intervals 0-25, 25-50, 50-75, 75-100, 100--125, 125-150 and 150-200 cm. Further augering to a depth of 450 cm was done to confirm that no groundwater table was present within that depth. In the soil samples it was determined that the soil material was a silty clay loam throughout the 200 cm depth. As the saturation percentages did not differ much in the horizontal and vertical directions, the electrical conductivity o f the extract of the saturated paste (ECe) was taken as a rough measure for the salt content of the soil. Hence the lines of equal electrical conductivity, drawn by linear interpolation in fig.2, may be
104
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SO[L TECHNOLOGY A ¢~loperating Journal of CATENA
Salt Traz]sport in CYacking Clay Soils (mI 2.015'
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go (m ECe:15-20 ;~ECe=20-1*O mmho/cm
Fig. 2: Vertical cross-section oJ'a large irrigated wheat fiehl on cracking, fine textured soils of the Khirtar Branch Command Area.
considered as rough isohalines.
5
4
To avoid the accumulation of salts in elevations, the fields would have to be levelled very accurately. Apart from the fact that precise levelling instruments would be needed for this, the implementation of the levelling is hampered by the very narrow workable moisture range of the soils. Moreover, the microtopography tends to reappear and strengthen itself. This is due to the fact that soil material, eroded in the depressions by the irrigation water, is transported in suspension through the cracks to the higher parts of the field. Unter intermittently irrigated crops the salt accumulation in elevations can be reduced by heavy irrigation applications that flood also the higher parts of the fields. However, the disadvantages of this is that it leads to crop damage by prolonged flooding in the depressions (photo 4). Only rice crops can tolerate such prolonged inundation.
Results and discussion
The results in fig.2 confirm that the salt distribution in the field is strongly related to height differences amounting to only a few centimeters over distances of 10 m or more. The occurrence of external solonchaks on the elevations proves that these locations receive irrigation water almost exclusively by horizontal bypass flow in the topsoil. Below depths of 40 to 60 cm, which is roughly the depth of the cracked surface soft, the soils are only moderately saline, which indicates that there is little leaching of salts from the cracked surface soil to the not cracked subsoil. Also in the depressions there appears a slight decrease of the salinity with increasing depth, which shows that also here the soil is moistened partly by capillary ascent from the bottom of the cracked zone. The general level of the salinity in these depressions is, however, much lower, which is due to more extensive leaching afterwards.
Recommendations
Acknowledgement Permission for publication of this paper was obtained from Hunting Technical Services Ltd., Thensfield House, Bound-
SOIL TECIINOLOGY
A coopcraling Journal of CA'I'ENA
106
Kamphorst
ary Way, Hemel Hempstead (Herts) England. References HOORN, J.W. VAN (1984): Salt transport in hea W clay soil. In: Proc. of the ISSS Symp. on Water and Salt movement in heavy clay soils. Int. Inst. for Land Reclamation and Improvement Publ. 37, Wageningen, Netherlands, 229240. KAMPHORST, A. (1988): Water and Salt transport in the irrigated cracking clay soils of the Kachhi Plains, Pakistan. I. Vertical transport. SOIL TECHNOLOGY vol. 1, nr. 3, 271-281, KAMPHORST, A. & EL BHANASAWY, N. (1988): Experimental laboratory simulation of the leaching of salt from an aggregate of a cracking clay soil. In: Proc. Int. Workshop on the Classification, Management and Use Potential of Swell-Shrink Soils, Nagpur, India, October 1988, 101-106.
Address of author: A. Kamphorst Department of Soil Science and Plant Nutrition Wageningen Agricultural Univesity Dreijenplein 10 6703 HB Wagcningen, The Netherlands
SOIL TECnNOLOGY--A cooperating Journal of CATENA
vol. 2, p. 101-106
COMMUNICATION [
WATER A N D SALT TRANSPORT IN THE IRRIGATED CRACKING CLAY SOILS OF THE KACHHI PLAINS, PAKISTAN PART II. HORIZONTAL TRANSPORT A. K a m p h o r s t , W a g e n i n g e n
Summary A description of the vertical transport of water and salts in the irrigated cracking clay soils of the Kachhi Plains of Pakistan (KAMPHORST 1988) is followed by an analysis of the horizontal transport components. During basin irrigation water is transported through the cracks of the topsoil, from oblong erosional microdepressions towards isolated elevations. This leads to the formation of external solonchaks on the elevated parts of the fields. As better levelling of the fields is difficult to achieve, leaching of the salts from the elevations by heavy irrigation applications is the only solution to this salt problem. Under intermittently irrigated crops, however, this is not quite possible, as it leads to water logging in the depressions. Therefore, it is recommended to grow a wet rice crop once in several years.
ISSN 0933-3630 @1989 by CATENA VERLAG, D-3302 Cremlingen-Destedt,W. Germany 0933-3630/89/5011851/US$ 2.00 + 0.25 SOIL TECHNOLOOY--A cooperating Journal of CATENA
1
Introduction
KAMPHORST (1988) showed that the vertical transport of water and salts in fine-textured topsoils of the Kachhi Plains in Pakistan takes place almost exclusively in the cracks. Due to the slow and incomplete closing of these cracks upon wetting of the soils, this "bypass" transport may continue for more than one month under continuous flooding. In deep fine textured soils the internal drainage and salt leaching are limited by the uncracked fine textured subsoils rather than by the cracked fine textured topsoils. In the situation just described the movement of free water through the inter-aggregate cleavages may take place not only in a vertical but also in a horizontal direction, particularly if the subsoil is impervious to water transport. Horizontal bypass flow in gently to moderately sloping black cotton soils in India was already mentioned in a companion paper (KAMPHORST 1988). The piping gullies in such soils are formed when rain showers fall on a dry, cracked surface soil. The water entering the cracks then runoff at the bottom of the cracked topsoil. Downslope the accumulated sub-
102
Kamphorst
surface runoff creates pipes or tunnels by erosion. U p o n their collapse these pipes turn into rills and, finally, gullies (photo 1). Not only in eroding landscapes but also in sedimentary plains were the effects of horizontal bypass flow in cracking clay soils observed. In the uncultivated Kachhi Plains, for instance, sedimentation takes place by "flash floods", which enter the piedmont plain from the hills after rainfall. When the flood water reaches the fine-textured soils it enters the crack system, where it deposits its entire sediment load. In this way the cracks sometimes are filled with sediment almost up to the soil surface (photo 2). VAN H O O R N (1984) reported preferential horizontal saturated transport of rain water towards drain trenches through highly permeable topsoils overlying slowly permeable clay layers in Spain and Portugal. The hig hydraulic conductivity o f the top layers was attributed to a better soil structure of the tilled layer in arable land and of the turf layer in grassland. This paper describes how horizontal transport through cracks takes place during basin irrigation o f the fine-textured Kachhi Plain soils. In that situation it influences not only the water distribution but also the salt patterns in the irrigated fields.
2
Background
Under flood or basin irrigation, water is generally applied to a field by making a breach in the bund of an irrigation channel. Due to this system water enters the field from one side, creating a wetting front that advances via the lowest parts towards the highest parts of the field. If the soil to be irrigated is an initally dry,
cracked clay soil, the water enters the crack system at the wetting front and then runs through the cracks towards the more distant parts of the field, as is shown in fig.1. The existence of such water distribution patterns in the cultivated fields of the heavy piedmont plain soils of the Khirtar Branch Command areas was confirmed by auger observations during the application of irrigation water. It was observed that the horizontal bypass flow took place concentrically from oblong depressions, created by microerosion, towards the higher parts of the fields in between. The lateral bypass flow shown in fig.1 implies that initial wetting of the soil in fact does not take place at the soil surface but at the bottom of the cracked topsoil. Due to this the infiltration of water into the prismatic soil aggregates has a horizontal concentric as well as an upward component. Hence the resident salts are washed towards the centre of the prisms ( K A M P H O R S T & B H A N A S A W Y 1988) and towards the soil surface. The latter component of the salt movement leads to the formation of external solonchaks, i.e., saline soils in which the salt content is highest at the soil surface and decreases with depth. As long as the subsoil below the cracked zone absorbs water, the concentric and upward capillary transport of water inside the prismatic aggregates is accompanied by downward bypass transport in the cracks. In principle this occurs at any point in the field, before as well as after the passage of the wetting front. This downward water transport is, however very inefficient with respect to salt leaching, as the salts are accumulated in the interiors of the prisms by the concentric flow.
SOIL TECHNOLOGY--A cooperating Journal of CATENA
Salt Transport in Cracking Clay Soils
103
N
r-77q77
X\/,
",~
\,,4
\'~/
"v
"",J
\,'
M
dry prismatic surface soil dry blocky subsurfuce soil wef prismafic surfQce soil wef blocky subsurface soil free wafer
Fig. 1: Schematic vertical cross-section, perpendicular to the irrigation front, of an
initially dry field on a cracking clay soil.
Appreciable leaching of salts can take place only where the downward transport lasts for many days after the prisms are wetted. During such a period the salts can return to the exteriors of the aggregates and from there to the bypass water by molecular diffusion ( K A M P H O R S T & BHANASAWY 1988). Long term flooding and hence vertical bypass transport of appreciable duration takes place only in the depressed parts of the fields. For instance, it was observed that inundation lasted for more than 1 week in the depressions of deep fine-textured soils after a heavy irrigation application, while the highest parts of the elevations were often not flooded at all. It is in these situations in particular that one should find external solonchaks on the elevations and non-saline or slighlty saline soils in the depressed parts of the irrigated fields.
3
Materials and methods
A large irrigated wheat field on deep fine textured soils, showing visible patches of
S O I L T E C H N O L O G Y - - A cooperating Joortaal of CATENA
salt damage (see photo 3), was selected to check the model described above. Seventeen auger holes to a depth of 200 cm were made along a straight line passing through several salt patches. At each auger site the level of the soil surface was determined with the aid of an accurate levelling instrument and the crop cover was recorded. From the auger holes soil samples were taken from the depth intervals 0-25, 25-50, 50-75, 75-100, 100--125, 125-150 and 150-200 cm. Further augering to a depth of 450 cm was done to confirm that no groundwater table was present within that depth. In the soil samples it was determined that the soil material was a silty clay loam throughout the 200 cm depth. As the saturation percentages did not differ much in the horizontal and vertical directions, the electrical conductivity o f the extract of the saturated paste (ECe) was taken as a rough measure for the salt content of the soil. Hence the lines of equal electrical conductivity, drawn by linear interpolation in fig.2, may be
104
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~.~
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~
~.~
~
"~ '~ ~
~.~
~
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.. ~,'~ e
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~.~
. . . ~ .~
~.~
SO[L TECHNOLOGY A ¢~loperating Journal of CATENA
Salt Traz]sport in CYacking Clay Soils (mI 2.015'
,
...
.
.
= ......
.
.
.
i[
.
105
.
,
i
.
.
.
.
= ,i
.
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I
(m) 2.0
l@~, t
.5
--
1.0
.0
0.5 ---.
i .
.
.
.
.
~
I
I
1
I
i1,11'11'1'1
Io LEG£NO: ~ = wheat cover; [ ~
I %111 i i
I I1,
2'0
I Irlll
i
I
IIii
~
0,5
~
3b
ECe= I-3 ;{ ] ~ ECe= 3-5; ~
4'o
EEe= 5-10 ;~
ECe=10-15; ~
go (m ECe:15-20 ;~ECe=20-1*O mmho/cm
Fig. 2: Vertical cross-section oJ'a large irrigated wheat fiehl on cracking, fine textured soils of the Khirtar Branch Command Area.
considered as rough isohalines.
5
4
To avoid the accumulation of salts in elevations, the fields would have to be levelled very accurately. Apart from the fact that precise levelling instruments would be needed for this, the implementation of the levelling is hampered by the very narrow workable moisture range of the soils. Moreover, the microtopography tends to reappear and strengthen itself. This is due to the fact that soil material, eroded in the depressions by the irrigation water, is transported in suspension through the cracks to the higher parts of the field. Unter intermittently irrigated crops the salt accumulation in elevations can be reduced by heavy irrigation applications that flood also the higher parts of the fields. However, the disadvantages of this is that it leads to crop damage by prolonged flooding in the depressions (photo 4). Only rice crops can tolerate such prolonged inundation.
Results and discussion
The results in fig.2 confirm that the salt distribution in the field is strongly related to height differences amounting to only a few centimeters over distances of 10 m or more. The occurrence of external solonchaks on the elevations proves that these locations receive irrigation water almost exclusively by horizontal bypass flow in the topsoil. Below depths of 40 to 60 cm, which is roughly the depth of the cracked surface soft, the soils are only moderately saline, which indicates that there is little leaching of salts from the cracked surface soil to the not cracked subsoil. Also in the depressions there appears a slight decrease of the salinity with increasing depth, which shows that also here the soil is moistened partly by capillary ascent from the bottom of the cracked zone. The general level of the salinity in these depressions is, however, much lower, which is due to more extensive leaching afterwards.
Recommendations
Acknowledgement Permission for publication of this paper was obtained from Hunting Technical Services Ltd., Thensfield House, Bound-
SOIL TECIINOLOGY
A coopcraling Journal of CA'I'ENA
106
Kamphorst
ary Way, Hemel Hempstead (Herts) England. References HOORN, J.W. VAN (1984): Salt transport in hea W clay soil. In: Proc. of the ISSS Symp. on Water and Salt movement in heavy clay soils. Int. Inst. for Land Reclamation and Improvement Publ. 37, Wageningen, Netherlands, 229240. KAMPHORST, A. (1988): Water and Salt transport in the irrigated cracking clay soils of the Kachhi Plains, Pakistan. I. Vertical transport. SOIL TECHNOLOGY vol. 1, nr. 3, 271-281, KAMPHORST, A. & EL BHANASAWY, N. (1988): Experimental laboratory simulation of the leaching of salt from an aggregate of a cracking clay soil. In: Proc. Int. Workshop on the Classification, Management and Use Potential of Swell-Shrink Soils, Nagpur, India, October 1988, 101-106.
Address of author: A. Kamphorst Department of Soil Science and Plant Nutrition Wageningen Agricultural Univesity Dreijenplein 10 6703 HB Wagcningen, The Netherlands
SOIL TECnNOLOGY--A cooperating Journal of CATENA