| Urban greenbelt plays an irreplaceable role in maintaining stability of urban ecosystem and improving economical and social sustainable development. In semi-arid regions characterized by water resources shortage, weak ecological environmental, and relatively backward social and economic developing level, not only urban greenbelt can improve the environment of public living,but promote foreign investment and social and economical development. However, rapid expansion of the urban greenbelt for the city in semiarid regions will bring new contradictions, that is, increasing water consumption in urban greenbelt irrigation.In this paper, aiming at rainwater harvesting from the sidewalk to apply the greenbelt, a new kind model of rainwater harvesting and utilization of urban greenbelt and sidewalk was presented, and in order to get the effects of rainwater harvesting in this model and the plant growth, field experiment was carried out for the model in Lanzhou city, located in semiarid region of northwest China. The experimental design include three kinds of the catchment area of rainwater harvesting, that is M1(2mx2m), M2(3mx3m) and M3(4mx4m) respectively. The surface of the catchment was lined by plastic film with0.8mm thick in order to raise water harvesting efficiency. Meanwhile, a non-rainwater harvesting greenbelt with Trifolium plant took as control (CK). The results are showed as follows:(1) The model for rainwater harvesting on sidewalkA new rainwater harvesting and utilization model was presented, which can be applied for sidewalk trees greenbelt, hedges, lawn or flower belts. The characteric of the model is changing the greenbelt to dot area and taking the dot area as a center chich has low level so that rainwater on the sidewalk flows to the area. Comparing the model with tranditional greenbelts of road two sides, the model designs a slightly undulating terrain in the sidewalks and drainage slopes to the dot area. the sidewalk acts as catchment for rainwater harvesting and the pattern is dot Greenland with rainwater harvesting.(2) The rainwater harvesting efficiency of the model43events of rainfall occured during the experiment period with total rainfall255.2mm. Three catchment areas of the model including M1, M2and M3with area of4m2,9m2and16m2collected rainfall906.1L,2035.8L, and3596.8L, respectively. The rain-runoff linear model indicated that with increasing of catchment area the critical rainfall of runoff generation and catchment efficiency of each treatment was decreased.(3) Soil water content variation in the three treatments with planted Trifolium repensRainwater harvesting treatments showed distinct contribution to soil water content of Trifolium repens Greenbelt. Soil water content at the depth from0to90cm in M1, M2and M3greenbelt all was higher than that in none rainwater harvesting greenbelt CK. With the increase of catchment area, the difference in soil water content became more significantly between rainwater harvesting greenbelts and control greenbelt. The soil water temporal and spatial patterns after single rainfall showed that the change of soil water at upper layers was greater and soil moisture at the depth from0to20cm decreased day by day. Soil water in rainwater harvesting greenbelts appeared clear infiltration process1-2days after rain, so the soil water content at the depth from50cm to90cm increased significantly. Soil water in each day and each layer showed significant difference among rainwater harvesting treatments, meanwhile the interaction of the two had distinct effect on soil water. Meanwhile, daily evapotranspiration increased accordingly in each treatment, daily evapotranspiration in control greenbelt was significantly lower than that in rainwater harvesting greenbelt. Daily evapotranspiration in M1was distinctly lower than that in M2and M3, and the difference in daily evapotranspiration between M2and M3was not significant.(4) Photosynthetic responses of Trifolium repens to the three treatments The average daily value of photosynthetic rate (Pn) and transpiration rate (Tr) of Trifolium repens in CK were low than every rain collecting treatment, and with the area of rain collecting increased Pn and Tr both increased; The water use efficiency (WUE) average daily value in CK was low than every rain collecting treatment, and among the rain collecting treatments, the value was maximum in M3and minimum in M2. In each treatment Pn decreased almost all showed Ci increased and Ls decreased, so the main cause of Pn decreasing in each treatment is non stomatal limitation. Treatment has no significant effect on WUE, but had very significant effect on other photosynthetic characteristics of Pn, Tr, gas conductance (Gs), interlellular CO2concentration (Ci) and stomatal limitation (Ls).(5) The responses of Trifolium repens growth to the three treatments Aboveground morphological character showed that the coverage of CK was significantly lower than that of rainwater harvesting greenbelts in each month and the coverage in each month differed not significantly among rainwater harvesting treatments. The coverage in CK reached100percent in September while the time was moved up to August for rainwater harvesting treatments. The projected area of grass crown in CK was significantly lower than that in M2and M3and had no obvious difference with M1, so the projected area of grass crown in M1was obviously lower than that in M2and M3and the latter two had no significant difference. Main branch length differed not significantly among rainwater harvesting treatments and it was only obviously lower in CK than that in M2. Length of first-class lateral branch in CK was clearly shorter than that in rainwater harvesting treatments with no significant difference in length of primary branch. Number of primary branch in CK was largest and had no marked difference compared with rainwater harvesting treatments and it also had no significant difference among the rainwater harvesting treatments. Number of the second-class branch in CK was markedly lower than that in rainwater harvesting treatments with no significant difference among them. Aboveground biomass in CK was significantly lower than that in rainwater harvesting treatments, aboveground biomass in M1was obviously lower than that in M2and M3, while the latter two had no significant difference, and the maximum value appeared in M2.(6) Root characteristics of Trifolium repens in the three treatmentsMorphological characteristics of root indicated that total root weight density, total root length density, total root surface area density and total root volume density of absorptive roots(≤1mm) and conductive roots(>lmm) were in the descending order of M2, CK, M1, M3. Vertical distribution characteristics of roots showed that vertical distribution of total root weight density of Trifolium repens in all green lands were basically consistent. The root weight density decreased with increasing of soil depth, the maximum value appeared at the depth from0to10cm and at the soil layer root weight density proportion of absorptive roots decreased and that of conductive roots increased with increasing of catchment area. The vertical of root length density, root surface area density and root volume density of absorptive and conductive roots took on basically the same trend, reduced with increasing of soil depth and reached the maximum value at the depth from0to20cm and they were higher in CK than those in rainwater harvesting treatments. The root length density, root surface area density and root volume density of absorptive roots in M2were highest compared with those in CK, M1and M3at the depth from30to90cm and those of conductive roots distributed intensively at the depth from0to20cm. The root-shoot ratio of Trifolium repens was in the descending order of CK, M2, M1, M3, it was distinctly higher in CK than that in rainwater harvesting treatments and it was significantly lower in M3than that in M1and M2, and the latter two had no marked difference in root-shoot ratio.The results of the above studies showed that the catchment efficiency of the new model of rainwater utilization of urban greenbelt designed in this paper is remarkable.The eco-hydrological effects of the rainwater harvesting greenbelts formed under the new model showed that to compared with non-rainwater harvesting greenbelt the soil water content increased significantly in rainwater harvesting greenbelts, and the water in deep soil body also increased, so the water supply of greenbelt plants during growing season could be ensured. The photosynthetic characteristics of Pn, Tr, Gs and Ls improved significantly but Ci reduced significantly, and the photosynthetic efficiency enhanced in rainwater harvesting greenbelts of Trifolium repens, so the drought stress phenomenon did not appear in plants in greenbelt having rainwater harvesting. The coverage closured in advance, the projected area of grass crown expanded, the length of first-class lateral branch became long, meanwhile the number of the second-class branch and the aboveground biomass increased significantly, so the ornamental and the landscape effect of the greenbelt became superior. In rainwater harvesting greenbelt of Trifolium repens, the proportion of root weight density, root length density, root surface area density and root volume density of absorptive roots decreased, meanwhile the proportion of root weight density, root length density, root surface area density and root volume density of conductive roots increased, and the Vertical distribution characteristics of root weight density, root length density, root surface area density and root volume density was the same as the non-rainwater harvesting greenbelt, and the root biomass increased while the root-shoot ratio decreased significantly, therefore, the harvesting rainfall amount had a significant influence on root biomass while had no significant influence on root distribution. In the three catchment area of the model, the variation of soil water content, photosynthetic indicators, morphological and biomass both in aboveground and underground showed that the amount of water collected by treatment M2, catchment area9m2, can ensure the plant growth and get the biggest biomass. |
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