Water and nitrogen transport characte-ristics of single-line interference infiltra-tion under film hole irrigation with muddy water and fertilizer

, ,

(State Key Laboratory Base of Eco-hydraulic Engineering in Arid Area, Xi′an University of Technology, Xi′an, Shaanxi 710048, China)

Abstract: Based on the experimental data, this study investigated the effect of sand content of muddy water on water and nitrogen transport characteristics of the single-line interference infiltration under film hole irrigation with muddy water and fertilizer. The relationship between the single-line interference infiltration parameters, the sand content, the wetting front movement distances, and the sand content were all established. The model of the cumulative infiltration volume of per unit film pore area, the vertical and horizontal wetting front movement distance of the free surface, and the wetting front movement distance of the interference center with sand content and infiltration time were proposed. Reveal the law of the change of soil water content and the distribution of content based on different muddy water sand content. The results indicate that at the same infiltration time, as the muddy water sand content increases, the cumulative infiltration volume per unit pore area decreases. The infiltration index of the free infiltration and the single-line interference vary little when the sand content increases, mainly are around 0.64 and 0.58. The relationship between infiltration parameters a, b and the sand content is linear function. At the same location, the more the sand content, the smaller the wetting front movement distance in free surface and the single-line interference surface, the less thecontent.

Key words: film hole irrigation;single-line interference infiltration;muddy water;fertilizer;

The film hole irrigation technology is a ground irrigation method in which the film is applied to the surface of the soil, and holes are formed in the film for irrigation. Film hole irrigation combines several water-saving irrigation methods such as plastic mulching, ground irrigation, and drip irrigation, which not only reduces water evaporation amount and improves irrigation efficiency, but also is widely used due to its simple operation and strong controllability[1-4]. Film hole infiltration is a three-dimensional point source infiltration under sufficient water supply conditions. Due to the difference in planting spacing, it can be divided into two types: the single-point film hole infiltration with larger spacing and the interference infiltration with smaller crop spacing[5-6].

At present, some domestic scholars have condu-cted relational researches on film hole irrigation infiltration and water-fertilizer coupling under clear water conditions[7-9], but under actual conditions, it is mainly irrigation of muddy water, especially in the Yellow River basin of China. Clear water and muddy water infiltration mechanisms are obviously different, so there are some differences in infiltration characteristics[10-11]. Some scholars at home and abroad have done a lot of researches on the water infiltration of muddy water film hole irrigation, mainly from its infiltration characteristics and influencing factors[12-13]. JIN, et al[14], FEI, et al[15], ZHONG, et al[12], etc, studied the effects of factors such as initial soil moisture, film-hole diameter, and muddy water sand content on the infiltration characteristics of the single-point film hole infiltration. The results show that: under the same conditions, the larger the initial soil moisture, the larger film-hole diameter, and the larger muddy water sand content, the smaller the cumulative infiltration volume per unit film hole area. MA, et al[16]studied the model for soil water movement of the film hole irrigation and proposed a simplified model of the average infiltration depth of the film hole irrigation including the hole opening rate and the film-hole dia-meter.

However, the researches on the single film hole infiltration of muddy water and fertilizer are little[17-18], and there has not been any research on the single-line interference infiltration of film holes with muddy water and fertilizer. The multi-point film hole interference infiltration is the research basis of film hole irrigation technology, muddy water irrigation and crop fertiliza-tion are inevitable measures in actual agricultural production. Therefore, the study of water and nitrogen transport in the single-line interference infiltration under muddy water film hole irrigation has important theoretical value and production practical significance. Under the research conditions of clear water film hole with fertilizer, the characteristics of single-line interfe-rence infiltration, soil moisture content and distribu-tion of fertilizer content under film hole irrigation with muddy water and fertilizer are studied.

1 Materials and methods

1.1 Laboratory experiments and method

In order to study the single-line interference infiltration characteristics of muddy water film holes with fertilizer under different muddy water sand content conditions, the experiment device is shown in Fig.1. This study is about the infiltration of muddy water, in order to prevent the precipitation of muddy water during the infiltration process from affecting the test results, a stirrer system is added to the marriotte bottle of the clear water infiltration device, and the muddy water was continuously stirred during the experiment to ensure a constant muddy water content. Considering the symmetry of film hole irrigation, we taken the 1/4 soil bin to study. The soil-filling bin used for the infiltration is made of transparent and organic glass, measured 24 cm in length, 10 cm in width, and 50 cm in depth. The purpose is to facilitate the observation of the change of the wetting front. The test is simulated using a quarter of the membrane hole, and the film hole is placed at a corner of the soil-filling bin. The film hole is made of transparent and organic glass with a thickness of 5 mm, the radius is 3 cm, and the height of the water head is 5 cm. The marriotte bottle is adju-sted to achieve the planned water head. The inner diameter of the marriotte bottlexis 70 mm, the diameter of the rotation axis is 14 mm, and the height is 90 cm. The volume of the blade is only 2‰ of the volume of the Markov flask, which can be ignored. The effect of soil specificity is ignored in the experiment. According to the characteristics of water infiltration, the interface of ″abcd″ shown in Fig.1 is a single-line interference interface produced by multipoint source infiltration. Infiltration in the other direction is borderless, and the movement of the wetting front is unlimited. For the convenience of research, only the right side of the interface of ″abcd″ is taken as the research object.

Fig.1 Experimental setup for single-line interference infiltration of muddy water film-hole irrigation with fertilizer

1.2 Experiment materials

The infiltration method is single-line interference infiltration of film hole irrigation. The fertilizer is cal-cium ammonium nitrate, the fertilizer is dissolved in muddy water and sent to the soil by irrigation. Muddy water sand content is the proportion of muddy water weight in muddy water weight.

The soil sample used in the experiment was Xi′an silt loam soil. The soil sample was air-dried and sieved a 2 mm mesh for using. Before the experiment, the related indexes of the soil samples were measured. The particle size distributions of the silt loam were 9.68% 0

2 Result analysis

2.1 Influence of muddy water sand content on infiltration capacities

Fig.2 is the graphs showing the change of cumulative infiltration volume per unit film-hole area of diffe-rent sand contents with infiltration time. It can be seen from the figures, other conditions are equal, the cumulative infiltration volume per unit film-hole area gra-dually increases with the infiltration time; at the same infiltration time, the greater the sand content of muddy water, the less the cumulative infiltration volume of per unit film-hole area. The reason is that during the infiltration process, some sands in muddy water were deposited on the soil surface and a dense layer was formed on the surface; in addition, a part of the sediment particles enter the soil with the infiltration water flow, blocking the pores with the water flow into the soil, forming a detention layer. Both the dense layer and the detention layer have hindered the infiltration of water flow. The volume of infiltration water was the same, and the infiltration time of muddy water (1%,3%,5%) increased by 17.71%, 34.35%, and 86.61%, respectively compared with the infiltration time of clear water.

Fig.2 Cumulative infiltration per unit film-hole at different sand content

Analyze the experimental data in Fig.2 and use the Kostiakov infiltration model to describe the relationship between the cumulative infiltration volume per unit film-hole area at each stage of infiltration and time, expressed as Eq.(1).

(1)

whereIis the cumulative infiltration volume per unit film-hole area, cm;aandeare infiltration parameters;banddare infiltration exponent;t0is the confluence time, min;tis the infiltration time, min.

The parameter values ofa,b,dandewere obtained from fittingIusing Eq.(1) for different infiltration times in Tab.1. In the free infiltration and the single-line interference infiltration stages, theaandegradually decrease with the increase of the sand content of muddy water, and thebanddchanged little, the values are around 0.64 and 0.58. The determination coefficients of the fitted results are all larger than 0.95, indicating that the relationship between the cumulative infiltration amount and the infiltration time is a Kostiakov infiltration model.

Tab.1 Fitted infiltration parameter values

The infiltration parameter was further analyzed, the relationship betweenaandγis a liner function:

a=-23.330γ+1.569 6,R2=0.978 5,

(2)

b=-23.136γ+1.823 2,R2=0.989 7.

(3)

Taking a significance level of 0.01, for the series containing 4 points, the critical correlation coefficientrd=0.900 0. From the fitting results of equations (2) and (3), we can know that the correlation coefficientsRare greater than the critical correlation coefficient. Therefore, the relationship between the sand contentγand the infiltration parameteraandbof the single-line interference infiltration of muddy water film hole with fertilizer in equations (2) and (3) can be used to determine the cumulative infiltration volume per unit film-hole area, which has high reliability.

2.2 Influence of muddy water sand content on law of wetting front movement

The muddy water sand content affects the infiltra-tion rate of muddy water and then has an important impact on the movement of the wetting front. The wetting front movement determines the distribution of water in the field and then affects the quality of irrigation. Therefore, analyzing the rate of the wetting front movement is an important indicator for studying the muddy water film hole infiltration of different sand content.

2.2.1 Wetting front of free surface

The influences of the sand content on the horizon-tal wetting front and the vertical wetting front with the infiltration time are shown in Fig.3. It is concluded from Fig.3 that the transport velocity of the horizontal and vertical wetting fronts gradually decreases as the increases of time. At the same time, the more the sand content, the shorter the wetting front movement distance, whether the horizontal direction or the vertical direction.

Fig.3 Influences of sand content on horizontal and vertical wetting front with infiltration time

When the water supply is started, the film hole is instantly filled with water, so the horizontal wetting front rapidly increases to the radius of the film hole. The vertical wetting front is smaller than the horizontal wetting front at the beginning of the infiltration. With the increase of the infiltration time, the vertical direction is affected by both the matrix potential and the gravity potential, while the horizontal direction is only affected by the matrix potential. Therefore, the transport velocity of the wetting front in the vertical direction is gradually greater than that in the horizontal direction. In the same infiltration time, the horizontal and vertical wetting front transport distance has a negative correlation with the sand content and decreases with the increase of the sand content.

Through research and analysis, the relationship between the wetting front movement distances of the horizontal direction and vertical direction and the infiltration time is a power function:

Fx=mtn,

(4)

Fz=ptq,

(5)

whereFxis the horizontal wetting front transport distances, cm;Fzis the vertical wetting front transport distances, cm;m,n,p,qare the fitting parameters;tis the infiltration time, min.

The equations (4) and (5) were used to fit the relationship between the wetting front transport distancesFx,Fzand the infiltration time. The fitting results are shown in Tab. 2. The determination coefficients of the fitting results in Tab. 2 are all greater than 0.95, indicating that the relationship between theFx,Fzandtis the power function. The fitting parametersmandpdecrease with the increase of muddy water sand content, which indicates that the larger the muddy water sand content is, the smaller theFxandFzare at the same infiltration time. While the changes ofnandqare not obvious.

Tab.2 Fitted transport parameter values

The relationship between the fitted parameter values ofmandpand the sand content of muddy water can expressed as equations (6) and (7):

m=-0.356 5γ2-1.463 8γ+2.182 4,R2=0.988 5,

(6)

p=-2.856 4γ2-3.237 7γ+2.217 0,R2=0.988 4.

(7)

Taking a significance level of 0.01, for the series containing 4 points, the critical correlation coefficientrd=0.900 0. From the fitting results of equations (6) and (7), we can know that the correlation coefficientsRare greater than the critical correlation coefficient. Therefore, the relationship between the sand contentγand the infiltration timetof the single-line interference infiltration of muddy water film hole with fertilizer in equations (4) and (5) can be used to determine the wetting front transport distance, which has high reliability.

Through the analysis of equations (6) and (7), it is known that when the sand content increases to a larger value, the parametersmandpwill show negative values, and the infiltration will also show ne-gative values, which obviously does not conform to the actual situation. According to the analysis of the water quality of the Yellow River and Jinghe River, the sand content in the two rivers is generally around 5%, and as muddy water flows along the channel, a part of the sediment will settle in the channel. The sand content is less than 5%. Therefore, using this model to study the process of muddy water infiltration is feasible.

2.2.2 Wetting front transport characteristics at interference center

Fig.4 are the curves of the movement distance of the wetting front in the vertical direction at the interfe-rence center of the interference interface, under the condition of the single-line interference infiltration of different sand contents. It is known from the curves that since the interference occurred, the transport distance of the central axis of the single-line interference gra-dually increased with the infiltration time increases; at the same infiltration time, the larger the muddy water sand content, the smaller the wetting front transport distance, that is, the smaller the wetting depth. The greater the transport velocity of the wetting front at the interference central, the more favorable the uniform distribution of water in the crop root system, thereby improving the irrigation quality.

Fig.4 Wetting front of interference center

Through analysis, the relationship between the wetting front movement distanceFzof the interference center and the infiltration timetis a logarithmic function:

Y=flnt+n,

(8)

wherefandnare fitting parameters.

It can be known from Tab.3 that the correlation coefficients are all greater than 0.9, indicating that the relationship between the wetting front transport distance and the infiltration time at the single-line interference center is a logarithmic function.

Tab.3 Fitted movement parameter values

At the same time, it can be seen from Tab.3 that the migration parameterfincreases with the increase of muddy water sand content, andndecreases with the increase of muddy water sand content. Through further analysis of the fitting parameters, it is known that the relationship betweenmandnand the muddy water sand content is a linear function. The results obtained are as follows:

f=66.589γ+8.335 9,R2=0.937 9,

(9)

n=-492.99γ-32.675,R2=0.966 1.

(10)

Therefore, the relationship betweenthe wetting front transport distance at the interference center and the infiltration time and muddy water sand content can express as equation (11).

Y=(66.589γ+8.335 9)lnt+ (-492.99γ-32.675).

(11)

2.3 Distribution laws of soil water

Fig.5 are the distribution curves of soil water contentθat the center of the film hole and the center of the single-line interference irrigation, after the single-line interference infiltration of muddy water film hole with fertilizer (his the soil depth, and downward is positive). Controlling irrigation time and other influencing factors in the experiment are the same. It is determined that after the infiltration, the difference of water content at the same position in the wetting soil is due to the effect of the sand contents. The greater the sand content in the infiltration fertilizer solution, the more deposited sands on the surface of the soil at the location of the film hole, the more the infiltration channel is blocked, and the infiltration velocity decreases, the amount of infiltration is reduced. Take the film hole as the origin of coordinates, after the single-line interfe-rence irrigation of the film hole irrigation with muddy water and fertilizer, the sand content is 0, 1%，3%，5%, the soil water content are 29.48%，28.46%，27.31%，26.53%.Based on the clear water, the reducing content of different sand content infiltration liquid fertilizer is 3.46%, 7.36%, 10.00%.

Fig.5 Distribution curves of soil water content at film-hole center and interference center

It can be seen from Fig.5a that the center of the film hole has a smaller slope in the soil depth of less than 5 cm, which indicates that the gradient of water content change is small. In the initial stage of membrane pore infiltration, the water infiltration head played a major role, the soil water potential effect was small, and the infiltration velocity was faster. In the infiltration process, liquid fertilizer infiltration needs to continuously pass through the channel at the film hole, so that the upper soil water content gradually increases and approaches the saturated water content of the soil. The gradient of soil water contents in the lower soil layer is large because the wetting soil in the lower layer mainly depends on the gravity of the water and the matric potential of the soil. The infiltration rate is slow, the water gradually infiltrates, and the new wetting soil has smaller water content.

In the experiment, the location where the wetting front first appeared at the surface of the single-line interference infiltration is 2-3 cm below the soil surface, not the soil surface. It can be seen from Fig.5b that the soil water content at the depth of 2-3 cm below the soil surface in the interference center is greater than that of the soil surface. The film hole infiltration, liquid ferti-lizer enter the soil from the film hole, vertical water infiltration is not restricted; Limited by film holes in the horizontal direction, water can only enter the lower layer of soil first and then transport to the surface of the soil under the action of soil water potential.

2.4 Influence of sand content on distribution of content in wetting soil

Fig.6 Distribution of content at film-hole center and interference center

The single-line interference infiltration under the film hole infiltration of muddy water and fertilizer, the nitrate nitrogen content is mostly concentrated in the range of the wet radius is less than 10 cm. Nitrate-nitrogen is mainly distributed in the root layer, which is beneficial to crop growth.

3 Conclusions

1) The single-line interference infiltration under the film hole infiltration with muddy water and fertilizer, the relationship between the cumulative infiltration volume of per unit film hole area and the infiltration time is the Kostiakov infiltration model, and the correlation is good.

2) The movement distance of the horizontal and vertical wetting fronts is negatively related to the sand content in the free surface. The relationship between the transport distance and the infiltration timetareFx=mtnandFz=ptq; the relationship betweenmandpand muddy water sand content is a binary function; the relationship between the wetting front transport distance and infiltration time at the interference center is logarithmic function; the relationship between the fitting parameters and muddy water sand content is a linear function, the functional relationships between the wet front migration distance, infiltration time, and sand content are determined.

3) The gradient of soil water content in the upper layer at the central axis of the film hole is small; the first place where the wetting front appears at the single-line interference surface is the point which the soil surface 2-3 cm in depth, where the soil water content is the largest at the surface of interference.