Variation in soil properties caused by irrigation and cultivation in the central Gezira of Sudan

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<ul><li><p>Soil &amp; Tillage Research, 13 (1989) 57-74 57 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands </p><p>Variation in Soil Propert ies Caused by Irrigation and Cultivation in the Central Gezira of Sudan </p><p>I.M. BURAYMAH 1 and R. WEBSTER 2 </p><p>1Soil Survey Administration, Wad Medani (Sudan) ~Rothamsted Experimental Station, Harpenden, Hertfordshire AL5 2JQ (Gt. Britain) </p><p>(Accepted for publication 13 June 1988) </p><p>ABSTRACT </p><p>Buraymah, I.M. and Webster, R., 1989. Variation in soil properties caused by irrigation and cul- tivation in the central Gezira of Sudan. Soil Tillage Res., 13: 57-74. </p><p>The variation in soil created by agriculture under irrigation is assessed by comparing the soil on two adjacent 3.75-ha plots on vertisol in the Sudan Gezira. One plot has been cultivated and irrigated by flooding for over 50 years; the other remained in its original natural condition. The topsoil (to 30 cm) was sampled at intervals of 6.25 m on a grid and the pH, electrical conductivity and sodium adsorption ratio measured. The sample variances were up to 3 times larger on the irrigated plot than on the natural one. A spatial analysis showed almost no spatial dependence on the natural plot but a fairly strong dependence after irrigation with marked anisotropy. Most variation occurred perpendicular to the present river channel, but there was also subsidiary short- range variation approximately in the direction of flooding. To illustrate these effects, statistical surfaces were estimated by simple kriging from the data and their variograms and displayed as block diagrams. </p><p>INTRODUCTION </p><p>The soil of any region, however small, varies from place to place. This can present difficulties for the farmer, who can overcome them by dividing his land into more homogeneous parcels or attempt to homogenize the topsoil at least by cultivation, or some combination of the two. Irrigation in arid regions often results in local waterlogging and accumulation of salt. It is difficult to apply irrigation water uniformly, especially by flooding. Slight unevenness of the land surface causes water to pond in some places and not others. Ploughing creates more or less large ridges and furrows, and it can be difficult to flatten these completely before irrigating. Therefore, although the farmer desires a uniform soil, his management may inevitably create variation. If this is in the form of local redistribution or accumulation of salt then the result can be serious. </p><p>We know of no long-term trials designed to assess the effect of irrigation </p><p>0167-1987/89/$03.50 1989 Elsevier Science Publishers B.V. </p></li><li><p>58 </p><p>farming on soil variation The experimental programme at the Wad Medani research station in the central Gezira, however, provides an opportunity to study the effect of irrigation on soil pattern. Large-scale irrigation farming began in the region in 1925 and was supported by experiments at Wad Medani. As part of the research, one plot of land was left in its native condition, while the rest was irrigated to grow a variety of crops. Thus, by comparing the unused plot with the adjacent land the effect of farming on soil variation could be assessed. This paper describes the results of a survey designed specifically to make this comparison. </p><p>THE GEZIRA IRRIGATION SCHEME </p><p>The Gezira (Arabic for island) is a triangular region, bounded by the White and Blue Nile rivers south of their confluence, Fig. 1. It is part of the Central Clay Plain of Sudan with a slope of only 0.02% It lies on alluvium at least 20- m thick derived from the Ethiopian Highlands. The soil is dark brown or grey </p><p>16 </p><p>14" </p><p>! </p><p>N </p><p>Jebel Aul ia </p><p>-: ~...: .~ ": ' ' . . .Y" .. </p><p>..%.-, </p><p>\ ":.4 20 40 60 "~( </p><p>i I I </p><p>I </p><p>KHARTOUM </p><p>Gezira I rr igated Area </p><p>Hasahe isa </p><p>0 100 "~ , Sennar </p><p>Nad Medani </p><p>i ~, ,ome. . i ] i I 32" 33" 34" </p><p>Fig. 1. Map of the Gezira, showing the extent of irrigation farming and the location of the sample plots. </p></li><li><p>59 </p><p>cracking clay (vertisol) with montmorillonite as the dominant clay mineral. The climate is hot and arid or semi-arid {mean annual temperature is 27C and mean annual rainfall ranges from 200 mm in the north to 450 mm in the south) and the natural vegetation consists of sparse Acacia spp. and grasses. </p><p>The Gezira Irrigation Scheme began as a small pilot scheme in 1911 on 105 ha at Taiba, 10-km north of Wad Medani. Water was pumped directly from the Blue Nile river. Its area was gradually increased by the installation of more pumps further south. Large-scale irrigation was started in 1925, when a dam was constructed at Sennar (Fig. 1), and today since the construction of the Roseries Dam further south, some 900 000 ha are irrigated by water from the Blue Nile. </p><p>Cotton is the principal economic crop. It is grown in rotation with wheat, groundnuts, sorghum and vegetables. For cotton, the land is ploughed to a depth of about 22 cm and then formed into ridges 22-cm high and 80-cm apart. For wheat, the land is disc-harrowed and then levelled, while for groundnuts and sorghum, it is disc-ploughed and ridged. Irrigation is by flooding at an approximate rate of 1000 m 3 ha- ' every 14 days for cotton and more variably for the other crops. Excess water is led away through surface drains. Waterlog- ging occurs locally in years of exceptionally heavy rain, but too infrequently to have needed changes to the drainage system (Farbrother, 1972). The quality of water from the Blue Nile is good. Mustafa (1973) recorded maxima in the electrical conductivity of 0.12-0.22 ms cm- ' at low water over a 6-year period, and maximum values of sodium-concentration and sodium-adsorption ratio (SAR) of 1.3 m.e. l - ' and 1.44 respectively. Nitrogen fertilizer, as urea, is broadcast for cotton at about 120 kg of N ha- 1. Wheat receives about 80 kg N ha- ', broadcast at sowing. No other fertilizers are applied. </p><p>Research on the dynamic behaviour of the soil under irrigation began si- multaneously with the establishment of the Gezira Scheme in 1925. Rai ( 1969 ) summarized the work. The results were conflicting, and Rai could not draw general conclusions on the effect of irrigation because there were no compari- sons between the irrigated land and that left in its natural state. </p><p>PROCEDURE </p><p>To study the long-term effect of this agricultural practice on the soil, we compared the soil on two plots of land at Wad Medani, the agricultural re- search station established shortly after the inception of the Gezira Scheme. One plot had been used, as described, for more than 50 years, the other had been preserved in its natural condition, unused. The soil is the Suleimi series, the typical dark cracking clay (vertisol) first described by Greene (1928) that covers more than 90% of the Gezira. It contains some 60% of clay throughout the profile with a cation exchange capacity of approximately 100 m.e. per 100 g of clay. Robinson et al. (1969) provided a full description and chemical anal- </p></li><li><p>60 </p><p>TABLE 1 </p><p>Sampling variances within circular supports of 1-m radius </p><p>Plot Property Mean Variance loglo </p><p>Mean Variance </p><p>Natural </p><p>Irrigated </p><p>pH 7.78 0.004250 EC 0.521 0.002650 -0.284 0.001086 (ms cm- l) SAR 3.26 0.2478 0.509 0.003998 pH 7.51 0.003833 EC 0.439 0.005300 - 0.362 0.001457 (mscm -1) SAR 2.03 0.06658 0.284 0.003288 </p><p>EC = electrical conductivity; SAR = sodium adsorption ratio. </p><p>ysis, and in the appendix we give the latest bench-mark description by Nachtergaele (1976) together with average chemical analyses for the series. </p><p>The two plots, each 250 150 m, were adjacent to one another, though on opposite sides of an irrigation canal, some 4 km from the Blue Nile, see Fig. 1. The irrigated plot was cultivated in the north-south direction and flooded from the southern end and drained to the north. </p><p>In 1985, the soil of the two plots was sampled to 30 cm on a square grid at 6.25-m intervals. Using a bucket auger of 8-cm diameter, 5 cores were taken at random in a circle of 1-m radius around each grid node. The cores for each node were bulked for analysis. The sampling was performed along 24 rows from south to north in each plot. In alternate rows the sampling interval was in- creased to 12.5 m. The grid on the irrigated plot had 41 columns, that on the natural plot had only 36. </p><p>In the laboratory the samples of soil were air-dried, crushed to pass 2 mm and analysed in batches. Each batch consisted of 31 samples by rows. The pH was determined in a 1:1 suspension of 0.01 molar CaC12. Electrical conductivity was measured in a 1:1 soil:water saturation extract. Soluble sodium, calcium and magnesium were measured on the same extract and the sodium adsorption (SAR) calculated from the relation </p><p>SAR = Na/x / (Ca + Mg)/2 </p><p>where Na, Ca and Mg are expressed as concentrations in m.e. l-1. To obtain an estimate of the variation within the sampling support, i.e. the </p><p>circles of 1-m radius, a further 60 samples of soil were taken at random within the 1-m circles around 12 randomly chosen grid nodes. These were analysed chemically as above and the average variances within circles calculated. The results are given in Table 1. </p></li><li><p>61 </p><p>Preliminary data analysis </p><p>The frequency distributions of the data were examined first. Figure 2 shows them plotted as histograms, and they are summarized in Table 1. The distri- butions of electrical conductivity are markedly skewed with long upper tails, and, as Fig. 2 shows, they are well fitted by log-normal curves. The sodium adsorption ratio of the irrigated soil is also fairly strongly skewed and it too appears to be log-normally distributed. On the natural plot the distribution is only slightly skewed, and the log-normal curve fits little better than the normal curve. The pH on both plots is approximately normally distributed. </p><p>Since the purpose of the study was to see how using the land has affected the soil pattern, it was important to stabilize the variances. The measurements of conductivity and sodium-adsorption ratio were therefore transformed to their common logarithms for further analysis and comparisons. Table 2 gives their means and variances on these scales. The pH is, of course, already a logarith- mic transformation. So by transforming the others, all were brought to loga- rithmic scales. </p><p>Table 2 shows that using the land has diminished the average pH and con- </p><p>Ndturol Irrigoted </p><p>i Z05 </p><p>i 7.25 7.45 Z55 7.85 8.05 7105 </p><p>pH </p><p>725 Z45 Z65 7 85 805 </p><p>Electrical conduct iv i ty </p><p>mS dm -~ </p><p>4 6 8 10 12 14 </p><p>1 2 3 4 5 6 7 1 2 3 4 5 6 7 </p><p>Fig. 2. Frequency distributions of the data. Normal curves are fitted to those of pH and log-normal curves to those of the other variables. </p></li><li><p>62 </p><p>TABLE 2 </p><p>Summary of statistics </p><p>Plot Property Number of Mean Variance Skewness loglo measurements </p><p>Mean Variance Skewness </p><p>Natural pH 645 7.72 0.007534 -0.12 EC 642 0.524 0.005230 1.47 -0.284 0.003209 0.65 (mscm -1) SAR 642 3.52 0.5388 0.48 0.537 0.008762 -0.72 </p><p>Irrigated pH 744 7.62 0.01186 -0.63 EC 744 0.501 0.01764 2.10 -0.312 0.009934 1.03 (ms cm -~) SAR 744 2.11 0.4051 1.07 0.306 0.016840 -0.22 </p><p>EC = electrical conductivity; SAR = sodium adsorption ratio. </p><p>ductivity somewhat and the sodium adsorption ratio more so. This is presum- ably because sodium has been leached by drainage after irrigation. The variation in all three properties is increased, however, and most markedly for conductivity. </p><p>Spatial analysis </p><p>The distribution of the variation in space was analysed by computing var- iograms of each property. With sampling on a regular square grid we could estimate semi-variances at regular intervals in two dimensions using the for- mula given by Webster (1985). Alternatively, we could regularize the semi- variances over angular sectors. For this paper we show the results in the latter form since it is easier to present the fitted models in this way. The computing formula is </p><p>1 m(h) f +h)} 2 </p><p>I where '2 (h) is the estimated semi-variance for points separated by the lag h, m (h) is the number of pairs of observations at that separation, and z (xl) de- notes the measured value of the soil property z at position xi in the plane. The lag h in this formula, is a vector with both distance and direction, and here embraces a small range in each. The distance was incremented in single units of the sampling intervals, i.e. 6.25 in, with a range of one interval, and the direction incremented in steps of 22.5 with a range of 22.5 . The results are shown in Figs. 3-5 for pH, electrical conductivity and sodium-adsorption ratio respectively. In each figure the variogram for the natural plot is presented above </p></li><li><p>63 </p><p>0.015 </p><p>O4 </p><p>"1" CL 0 .010 </p><p>0 .005 </p><p>0 </p><p>, : - - . -~- I - | - j -~- - - : -T ', ! , o ~- ~ o o o o </p><p>o o </p><p>o 0 </p><p>Natural </p><p>I I I I 1 I I I </p><p>0.020 </p><p>0 .015 </p><p>O4 </p><p> ~ 0.010 </p><p>0 .005 </p><p>Directions </p><p>o N-S E -W </p><p> NNW-SSE WSW-ENE </p><p>V NW-SE E] SW-NE </p><p> WNW-ESE A SSW-NNE </p><p>| } - - o </p><p>- - 0 0 </p><p>0 0 </p><p>0 </p><p>__ - ; - J - - - t, X </p><p>I </p><p>I rr igated </p><p>J I I I I 1 I I 0 25 50 75 100 </p><p>Lag/m </p><p>Fig. 3. Semi-variograms of pH with different symbols for the eight directions. The lines show the fitted models (from Table 2 ). </p></li><li><p>64 </p><p>0.010 </p><p>Od #, E 0 D E o </p><p>0 v </p><p>;&gt;- </p><p>0.008 </p><p>0.006 </p><p>0.004 </p><p>0 - Natural </p><p>0.012 </p><p> __m_ </p><p>0.010 _m- m- - - ' - l - I -~e--vo v </p><p> ! ; L! - - ~" 0.008 ~.- - ~ [] : o . . - , _ _ - o z~ 0 </p><p>~ u." - = o </p><p>e o.oo8 ! O </p><p>O o n </p><p>" 0.004 o </p><p>0.002 Irr igated </p><p>I I I I I I I I 0 25 50 76 100 </p><p>Lag/m </p><p>Fig. 4. Semi-variograms of electrical conductivity. The symbols for eight directions are shown in Fig. 3, and the lines are those of the models specified in Table 2. </p><p>that of the irrigated one. The semi-variances for the eight directions are dis- played with distinguishing symbols. </p><p>The variograms show immediately the larger variances on the irrigated land. For pH this appears only at the longer lags: there is little difference between the two up to 25 m (4 lag units). For conductivity and SAR, however, the differences are manifest at all lags. </p><p>A second difference in soil between the used and unused is the auto-corre- lation. At the scale of investigation, there is almost none for pH and conduc- tivity on the natural plot. The semi-variance are on average much the same at </p></li><li><p>0.015 - </p><p>65 </p><p>~ 0.010 </p><p>~" 0.005 </p><p>. | _ - - - -= ' - 'T - 9 </p><p>- - "5~ - g ~ - V V </p><p>. . . . A /h </p><p>o </p><p>o </p><p>Natura l </p><p>I I 1 1 I I I I </p><p>0.025 </p><p>0.020 </p><p>o~ 0 .015 IZ </p><p>~- 0.010 </p><p>0 .005 </p><p>. ~"| o v X ~ ~ - 'e v ~ n A W J . </p><p>- -~- -0 - - 0 0 0 </p><p>o o </p><p>O </p><p>I r r igated </p><p>I I I I I I I 1 0 25 50 75 100 </p><p>Lag /m </p><p>Fig. 5. Semi-variograms of sodium adsorption ratio. The symbols for eight directions are as shown in Fig. 3, and the lines are the models specified in Table 2. </p><p>all lag distances: in the language of the subject the variograms are almost "pure nugget". The semi-variance of SAR increases somewhat with increasing dis- tance. This behaviour contrasts with that on the irrigated plot where there is a fairly marked auto-correlation. </p><p>The third feature of the variograms on the irrigated plot is that they show anisotropy: variation is substantially...</p></li></ul>

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