- / + random-float在netlogo中给出一个更改和固定的数字

Question

嗨伙计 我希望我的补丁变量 WTD 在过程浇水决策中增加了龟背景中的数量。但如果我写减号（ - ）随机浮动它会改变。否则显示恒定值。请帮助。

    globals [TW  well-depth]
    Breed [farmers farmer]
    farmers-own [ WA sw FW WR  dws Logging salinity
    ] 
    patches-own [
    GW wtd ] ;water table depth and Groundwater
    to setup
       clear-all
        setup-farmers
        ask patches [
        set WTD depth-WT + 10
        set GW Ground-water + random-float 5.005  +  (100 / depth-wt)
        ]

      reset-ticks
      end
    to setup-farmers
    create-farmers num-farmers [move-to one-of patches
                                 set shape "person"
                                 set sw surface-water + random-float 5.0
                                 set dws random 100 + distance-from-water-source
                                set wr minwater-requirement + random 10
                                set logging  0
                                set salinity  0
                                ask patches in-radius 1 [set pcolor green]
                                set color Brown
                                       ]
 end
    to go      
      ask farmers [
                 check-wa
                 **watering-decisions**
                 update-water]
                 tick
    end
    to check-wa
    SET WA  (sw / distance-from-water-source) + sum [gw] of patches in-radius 
     end
      **to watering-decisions**
      if watering-decision = "zero-intelligence"
      [
        if wr > 0 [set wa wa - random-float 3
        set FW wa
        set logging  0.5 * dws + wtd / 0.5
        set salinity   1.5 * dws + wtd * 1.5
        set logging logging + 0.005
        set salinity salinity + 0.001
        ask patches in-radius 1 [set pcolor green
        **set wtd wtd -  random-float 0.9** ]
        set color red    
        ]]

      end

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