Simulation of land use zoning optimization under multi-objective scenarios based on maximizing carbon storage: taking Qingshui River of Xijiang River in Guangxi as an example
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摘要:
土地利用变化是影响碳固存变化的重要因素,土地利用优化对实现区域碳平衡具有重要作用。以广西西江清水河流域为研究对象,基于2000年、2010年和2020年土地利用数据,通过FLUS-InVEST耦合模型预测2060年清水河流域4种模拟情景(基线情景、耕地保护情景、水域保护情景、高碳储用地保护情景)下土地利用变化与碳储量的时空发展特征;针对高、中、低碳储能力等级区域适宜发展的方向,构建基于碳储量最大化的灰色线性规划模型,优化土地利用数量结构并运用FLUS模型模拟土地利用空间布局;利用Fragstats软件分析流域上、中、下游区域不同土地利用类型的形态格局,探讨其与碳储量的相关性并提出相应的优化策略。结果表明:1)4种模拟情景下,2060年流域碳储量仅在高碳储用地保护情景下稳定提升,其他3种情景都大幅下降;2)基于优化方案,2060年流域内林地、湿地和水域面积增加,建设用地面积稳定增长,草地、耕地面积相对减少且连片耕地保持不变,流域整体碳储量增长达1.32×106 t;3)流域土地利用形态格局影响碳储量,且不同流段存在空间异质性,整体上斑块呈现复杂不规则的形态和较高的聚集度、连接度,有利于提高区域整体碳储量。优化策略能更好地满足流域不同区域的发展需求并统筹流域整体发展,增加流域碳储量的同时推动总体效益最优化。
Abstract:Land use change is an important factor affecting carbon sequestration change, and land use optimization plays an important role in realizing regional carbon balance. Based on the data of land use in 2000, 2010 and 2020, the temporal and spatial development characteristics of land use change and carbon storage in Qingshui River basin in 2060 were predicted by FLUS-InVEST coupling model under different simulation scenarios (baseline scenario, cultivated land protection scenario, water area protection scenario, and high-carbon storage land protection scenario). Aiming at the suitable development direction of high, medium and low carbon storage capacity grade regions, a grey linear programming model based on the maximization of carbon storage was constructed to optimize the quantitative structure of land use and simulate the spatial layout of land use using FLUS model. Fragstats software was used to analyze the morphological pattern of different land use types in the upper, middle and lower reaches of the basin, and analyze their correlation with carbon storage, and corresponding optimization strategies. The results showed that: 1) Under the four simulation scenarios, the carbon storage in the basin would increase steadily in 2060 only under the high-carbon storage land protection scenario, and decrease significantly under the other three scenarios. 2) Based on the optimization scheme, in 2060, the area of forest land, wetland and water area in the basin would increase, the area of construction land would increase steadily, the area of grassland and cultivated land would decrease relatively and the contiguous cultivated land would remain unchanged, and the overall regional carbon storage would increase by 1.32×106 t. 3) The land use pattern of the basin affected the carbon storage, and there was spatial heterogeneity in different segments. On the whole, the patches showed complex and irregular shapes with high degree of aggregation and connectivity, which was conducive to improve the overall carbon storage in the region. Therefore, the optimization proposed in this thesis can better meet the development needs of different regions of the basin while coordinate the overall development at the same time, and eventually increase the carbon storage of the basin and promote the overall benefit optimization.
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表 1 清水河流域不同土地利用类型各部分碳密度
Table 1. Carbon density of different land use types in Qingshui River basin
t/hm2 土地利
用类型地上部分碳
密度(Cax)地下部分碳
密度(Cbx)土壤碳密
度(Csx)死亡有机碳
密度(Cdx)耕地 19.43 3.85 51.31 1.06 林地 118.08 36.29 138.24 12.81 草地 3.19 14.33 53.42 1.44 湿地 53.28 17.05 58.76 4.32 水域 6.14 8.89 19.48 0.00 建设
用地5.93 0.98 4.02 0.00 表 2 4种情景下土地利用转换成本矩阵
Table 2. Land use conversion cost matrix under four scenarios
土地
利用
类型BD情景 CP情景 WP情景 HCP情景 A B C D E F A B C D E F A B C D E F A B C D E F A 1 1 1 1 1 1 1 0 0 0 0 0 1 1 1 1 1 1 1 1 0 1 0 1 B 1 1 1 1 1 1 1 1 0 0 1 1 0 1 0 1 1 0 0 1 0 0 0 0 C 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 D 1 1 1 1 1 1 1 1 0 1 1 1 0 0 0 1 1 0 0 1 0 1 0 0 E 1 1 1 1 1 1 1 0 0 0 1 1 0 0 0 0 1 0 1 1 1 1 1 0 F 0 0 0 0 0 1 0 0 0 0 0 1 0 0 0 0 0 1 0 1 0 0 0 1 注:A~F分别对应耕地、林地、草地、湿地、水域、建设用地。 表 3 4种情景下邻域因子参数
Table 3. Neighborhood factor parameters in four scenarios
土地利用类型 不同情景领域作用权重 BD情景 CP情景 WP情景 HCP情景 耕地 0.7 1 0.5 0.3 林地 0.5 0.5 0.5 1 草地 0.1 0.5 0.5 0.5 湿地 0.2 0.5 0.5 0.8 水域 0.4 0.5 1 0.2 建设用地 1 0.5 0.5 0.1 表 4 3 个碳储能力区各土地利用类型相应的约束条件
Table 4. Corresponding constraint conditions of each land use type in the 3 carbon storage capacity areas
约束类型 约束条件/km2 说明 总面积 $\begin{array}{l}{Z}_{t}=388.230\\ {Z}_{x}=1\;232.190\\ {Z}_{y}=2\;312.710\end{array}$ 根据土地面积平衡原理,在行政区划不发生改变的情况下,优化后流域3个碳储能力区面积前后保持不变,高(t)、中(x)、低(y)碳储能力区的总面积分别为388.23、1 232.19、2 312.71 km2 耕地 $\begin{array}{l}4.828\leqslant {t}_{1}\leqslant 7.935\\ 159.758\leqslant {x}_{1}\leqslant 237.232\\ {y}_{1}\leqslant 1\;865.957\end{array}$ t1和x1以CP情景下的耕地面积作为上限;y1则以保障优质连片耕地[28]为发展目标,严格落实耕地和基本农田保护任务,加快推进坡度25°以上的耕地和石漠化敏感区退耕还林还草 林地 $\begin{array}{l}{t}_{2}\geqslant 378.375 \\ 963.187\leqslant {x}_{2}\leqslant 971.031\\ 57.854\leqslant {y}_{2}\leqslant 67.680\end{array}$ t区和x区为生态涵养区和流域上游源头,应严格执行生态保护与修复要求,保障林地只增不减;y区中,将固碳能力较差的林地作为建设用地和耕地调入储备区。t2和x2至少不能低于HCP情景下的林地面积;y2则以BD情景为下限且不超过现有发展水平 草地 $\begin{array}{l}0.629\leqslant {t}_{3}\leqslant 0.633\\ 4.835\leqslant {x}_{3}\leqslant 5.230\\ 0.650\leqslant {y}_{3}\leqslant 0.683\end{array}$ 清水河流域草地面积较少,对其采取保护措施;草地的碳储能力相比林地较低,故t3和x3以HCP情景下草地面积为基础发展,加强遏制草地退化速率且提高部分地形坡度较大或固碳能力较差的草地向林地的转换率;y3则以2020年草地面积为发展下限 湿地 $\begin{array}{l} {t}_{4} > 0.100 \\ {x}_{4}\geqslant 0.102\\ {y}_{4}\leqslant 1.070\end{array}$ 湿地是重要的碳固存用地及研究区内面积最小的用地类型,故以保护与增加湿地为发展导向,t4和x4以WP情景模拟发展;y4则以WP情景草地面积为发展下限 水域 $\begin{array}{l}{t}_{5}\geqslant 0.500\\ 13.516\leqslant {x}_{5}\leqslant 32.800\\ 69.478\leqslant {y}_{5}\leqslant 70.992\end{array}$ t5位于清水河源头与上游地带,以2020年为保有量及WP情景为发展目标,发挥清水河流域优质水资源优势;x5和y5以保障现有水域面积为基础,以BD情景水域面积发展为上限 建设用地 $\begin{array}{l}0.216\leqslant {t}_{6}\leqslant 0.221\\ 11.460\leqslant {x}_{6}\leqslant 11.661\\ 221.739\leqslant {y}_{6}\leqslant 374.596\end{array}$ 清水河流域2000—2020年建设用地大幅度扩张,在保护资源和保持发展的前提下,t6和x6以HCP情景为保有量且不超过2020年现有发展面积;y6以BD情景发展为上限 表 5 2020年流域各类土地利用现状与2060年各类土地利用模拟情况
Table 5. Land use status in 2020 and simulations of different land use in 2060
土地利用
类型2020年现状 2060年预测值 BD情景 CP情景 WP情景 HCP情景 面积/km2 占比/% 面积/km2 占比/% 面积/km2 占比/% 面积/km2 占比/% 面积/km2 占比/% 耕地 2198.08 55.890 2041.27 51.900 2223.58 56.530 2036.63 51.780 2189.76 55.670 林地 1410.68 35.870 1343.09 34.150 1308.33 33.260 1398.57 35.560 1422.69 36.170 草地 6.56 0.167 5.96 0.151 6.00 0.152 6.10 0.155 6.18 0.157 湿地 1.16 0.029 1.19 0.030 1.11 0.028 1.17 0.030 1.14 0.029 水域 83.08 2.110 103.83 0.030 73.12 1.860 122.32 3.110 80.11 2.040 建设用地 233.56 5.940 437.78 11.130 320.98 8.160 368.33 9.360 233.25 5.930 表 6 2060年4种情景下清水河流域碳储量、平均碳密度及变化
Table 6. Carbon storage, average carbon density and changes in Qingshui River Basin under four scenarios in 2060
模拟情景 预测碳
储量/×106 t平均碳密
度/(t/hm2)2020—2060年
碳储量变化/×106 tBD情景 57.36 145.83 −2.96 CP情景 57.44 146.05 −2.87 WP情景 59.01 150.04 −1.31 HCP情景 60.61 154.10 0.29 表 7 清水河流域2020年现状与2060年碳储量最大化的多目标土地利用优化方案对比
Table 7. Comparison of multi-objective land use optimization scenarios for maximizing carbon stocks in Qingshui River basin in 2020 and 2060
土地利用类型 2020年现状 2060年综合优化模拟情景 面积/km2 碳储量/106 t 面积/km2 碳储量/106 t 耕地 2198.08 16. 63 2030.54 15.36 林地 1410.68 43.08 1492.15 45.57 草地 6.56 0.05 6.15 0.04 湿地 1.16 0.02 1.27 0.02 水域 83.08 0.29 85.01 0.29 建设用地 233.56 0.26 318.00 0.35 合计 3933.12 60.32 3933.12 61.64 表 8 清水河流域土地利用形态格局与碳储量相关性
Table 8. Correlation between land use pattern and carbon storage in Qingshui River basin
不同流段的土地利用类型 景观格局指数与碳储量的Pearson相关性指数 CA ED LSI PLADJ IJI COHESION DIVISION AI 耕地 上游 1.000** −0.452 0.689* 0.445 −0.653* 0.403 0.286 0.215 中下游 0.987** −0.541** 0.494** 0.707** 0.331 0.593** 0.012 0.520** 林地 上游 1.000** 0.125 0.394 0.972** 0.116 0.853** −0.722** 0.981** 中下游 0.966** 0.244 0.607** 0.633** 0.031 0.587** −0.527** 0.738** 水域 上游 1.000** −0.418 −0.151 0.918** 0.676* 0.927** −0.664* 0.920** 中下游 0.983** −0.107 0.766** 0.251 −0.106 0.296 0.283 0.08 建设用地 上游 1.000** −0.359 0.905** 0.641* 0.256 0.575 0.480 0.429 中下游 1.000** 0.105 0.837** 0.747** −0.708** 0.723** −0.032 0.282 草地 1.000** 0.959** 0.916** −0.746* 0.106 −0.412 0 −0.794* 湿地 1.000** 0.457 0.495 0.591* 0.572 0.583* 0 0.311 注:**表示P<0.01;*表示P<0.05。 表 9 清水河流域碳储量提升的土地利用形态格局发展特征分析
Table 9. Analysis of the correlation between carbon storage and land use patterns in Qingshui River basin
不同流段的土地利用类型 土地利用形态格局发展特征 耕地 上游 斑块大;形态复杂、不规则;斑块邻接类型少 中下游 斑块大且较聚集;形态复杂、不规则且边缘破碎度低;斑块连接度高 林地 上游 斑块大且较聚集;斑块连接度高 中下游 斑块大且较聚集;形态复杂、不规则;
斑块连接度高水域 上游 斑块大且较聚集;斑块连接度高且邻接类型多 中下游 斑块大;形态复杂、不规则;斑块连接度高、
聚集度较低建设用地 上游 形态复杂、不规则;斑块聚集度高 中下游 形态复杂、不规则;斑块聚集度、连接度高 草地 斑块大;不规则;斑块聚集度低 湿地 斑块大;斑块聚集度、连接度高 -
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